diff --git a/t/t7529-sparse-and-split-index.sh b/t/t7529-sparse-and-split-index.sh
new file mode 100755
index 00000000000000..b2f4825a61e9b5
--- /dev/null
+++ b/t/t7529-sparse-and-split-index.sh
@@ -0,0 +1,408 @@
+#!/bin/sh
+
+test_description='sparse-index and sparse-checkout are incompatible'
+
+. ./test-lib.sh
+
+r="repo-1"
+test_expect_success 'repo-1 : start with split-index and then enable sparse-index' '
+	git init $r &&
+	(
+		cd $r &&
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		# Try to turn on sparse-checkout and -index. Technically, this
+		# should fail because we already have split-index turned on.
+		#
+		# TODO Set this to test_must_fail ...
+		git sparse-checkout init --cone --sparse-index &&
+
+		#################################################################
+		# Everything below here is flakey because of the bad state that
+		# we have created. The following errors have been observed, but
+		# others may be possible.
+		#################################################################
+
+		# After creating an incompatible state, the first status USUALLY throws a
+		# BUG() assert.
+		#
+		# BUG: cache-tree.c:908: directory 'one/' is present in index, but not sparse
+		test_expect_code 134 git status >../$r.out 2>../$r.err &&
+		grep -e "BUG: .* is present in index, but not sparse" ../$r.err &&
+
+		# Debug dump enough state to show that both think they are turned on.  Oddly
+		# enough something causes the cache-tree error to go away.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		git status &&
+		ls -la &&
+
+		rm -f .git/index.lock &&
+
+		# Try to populate some cones in the worktree.  This fails in a weird way.
+		test_expect_code 128 git sparse-checkout set two three >../$r.out 2>../$r.err &&
+		grep "error: unable to create file three/: File exists" ../$r.err &&
+		grep "error: unable to create file two/: File exists" ../$r.err &&
+		grep "fatal: index cache-tree records empty sub-tree" ../$r.err &&
+
+		# Debug dump enough state to show what changed.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		ls -la
+	)
+'
+
+r="repo-2"
+test_expect_success FSMONITOR_DAEMON 'repo-2 : start with split-index and fsmonitor and then enable sparse-index' '
+	git init $r &&
+	test_when_finished "git -C \"$PWD/$r\" \
+		fsmonitor--daemon stop" &&
+	(
+		cd $r &&
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		git config core.fsmonitor true &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		# Try to turn on sparse-checkout and -index. Technically, this
+		# should fail because we already have split-index turned on.
+		#
+		# TODO Set this to test_must_fail ...
+		git sparse-checkout init --cone --sparse-index &&
+
+		#################################################################
+		# Everything below here is flakey because of the bad state that
+		# we have created. The following errors have been observed, but
+		# others may be possible.
+		#################################################################
+
+		# After creating an incompatible state, the first status USUALLY throws a
+		# BUG() assert.
+		#
+		# BUG: cache-tree.c:908: directory 'one/' is present in index, but not sparse
+		test_expect_code 134 git status >../$r.out 2>../$r.err &&
+		grep -e "BUG: .* is present in index, but not sparse" ../$r.err &&
+
+		# Debug dump enough state to show that both think they are turned on.  Oddly
+		# enough something causes the cache-tree error to go away.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		git status &&
+		ls -la &&
+
+		rm -f .git/index.lock &&
+
+		# Try to populate some cones in the worktree.  This fails in a weird way.
+		test_expect_code 128 git sparse-checkout set two three >../$r.out 2>../$r.err &&
+		grep "error: unable to create file three/: File exists" ../$r.err &&
+		grep "error: unable to create file two/: File exists" ../$r.err &&
+		grep "fatal: index cache-tree records empty sub-tree" ../$r.err &&
+
+		# Debug dump enough state to show what changed.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		ls -la
+	)
+'
+
+r="repo-3"
+test_expect_success 'repo-3 : start with split-index and then enable sparse-index' '
+	git init $r &&
+	(
+		cd $r &&
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		# Try to turn on sparse-checkout but NOT sparse-index. Technically, this
+		# should fail because we already have split-index turned on.
+		#
+		# TODO Set this to test_must_fail ...
+		git sparse-checkout init --cone --no-sparse-index &&
+
+		#################################################################
+		# Everything below here is flakey because of the bad state that
+		# we have created. The following errors have been observed, but
+		# others may be possible.
+		#################################################################
+
+		# Debug dump enough state to show what is turned on.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		git status &&
+		ls -la &&
+
+		rm -f .git/index.lock &&
+
+		# Try to populate some cones in the worktree.
+		#
+		# This creates the expected files in both directories.
+		git sparse-checkout set two three &&
+
+		# Debug dump enough state to show what changed.
+		git config --list --show-origin &&
+		git ls-files --debug &&
+		git ls-files --sparse &&
+		git ls-files -t &&
+		ls -la &&
+
+		# Now if we ad hoc add sparse-index things get weird.  The first status
+		# succeeds, but corrupts the index which causes the second to fail hard.
+		git -c index.split=true status &&
+		git -c index.split=true status >../$r.out 2>../$r.err
+
+#		test_expect_code 128 git -c index.split=true status >../$r.out 2>../$r.err &&
+#		grep "fatal: index cache-tree records empty sub-tree" ../$r.err
+	)
+'
+
+r="repo-4"
+test_expect_success FSMONITOR_DAEMON 'repo-4 : start with split-index and fsmonitor and then enable sparse-index' '
+	git init $r &&
+	test_when_finished "git -C \"$PWD/$r\" \
+		fsmonitor--daemon stop" &&
+	(
+		cd $r &&
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		git config core.fsmonitor true &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		echo x >initial/x1 &&
+		echo x >initial/x2 &&
+		echo x >initial/x3 &&
+		echo x >initial/x4 &&
+		echo x >initial/x5 &&
+		echo x >initial/x6 &&
+		echo x >initial/x7 &&
+		echo x >initial/x8 &&
+		echo x >initial/x9 &&
+		echo x >initial/y1 &&
+		echo x >initial/y2 &&
+		echo x >initial/y3 &&
+		echo x >initial/y4 &&
+		echo x >initial/y5 &&
+		echo x >initial/y6 &&
+		echo x >initial/y7 &&
+		echo x >initial/y8 &&
+		echo x >initial/y9 &&
+		git add . &&
+		git commit -m "ballast" &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		git rev-parse --shared-index-path &&
+		# TODO verify line count of 1 on the above
+
+		# Try to turn on sparse-checkout but DO NOT pass --sparse-index
+		#
+		# TODO Set this to test_must_fail ...
+		git sparse-checkout init --cone &&
+
+		#################################################################
+		# Everything below here is flakey because of the bad state that
+		# we have created. The following errors have been observed, but
+		# others may be possible.
+		#################################################################
+
+		# The following may succeed on upstream Git, but fails on microsoft/git
+		# since the latter assumes index.sparse=true
+		#
+		test_expect_code 128 git -c index.sparse=true rev-parse --shared-index-path >../$r.out 2>../$r.err &&
+		grep -e "fatal: position for replacement .* exceeds base index size .*" ../$r.err
+	)
+'
+
+r="repo-5"
+test_expect_success FSMONITOR_DAEMON 'repo-5 : start with split-index and fsmonitor and then enable sparse-index' '
+	git init $r &&
+	test_when_finished "git -C \"$PWD/$r\" \
+		fsmonitor--daemon stop" &&
+	(
+		cd $r &&
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		git config core.fsmonitor true &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		echo x >initial/x1 &&
+		echo x >initial/x2 &&
+		echo x >initial/x3 &&
+		echo x >initial/x4 &&
+		echo x >initial/x5 &&
+		echo x >initial/x6 &&
+		echo x >initial/x7 &&
+		echo x >initial/x8 &&
+		echo x >initial/x9 &&
+		echo x >initial/y1 &&
+		echo x >initial/y2 &&
+		echo x >initial/y3 &&
+		echo x >initial/y4 &&
+		echo x >initial/y5 &&
+		echo x >initial/y6 &&
+		echo x >initial/y7 &&
+		echo x >initial/y8 &&
+		echo x >initial/y9 &&
+		git add . &&
+		git commit -m "ballast" &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		git rev-parse --shared-index-path &&
+		# TODO verify line count of 1 on the above
+
+		# Try to turn on sparse-checkout.
+		#
+		# TODO Set this to test_must_fail ...
+		git sparse-checkout init --cone &&
+
+		#################################################################
+		# Everything below here is flakey because of the bad state that
+		# we have created. The following errors have been observed, but
+		# others may be possible.
+		#################################################################
+
+		git config --list --show-origin &&
+
+		# Sometimes the exit status is 128 and sometimes it is 134.
+		(
+			git ls-files --sparse >../$r.out 2>../$r.err ||
+			grep -e "fatal: position for replacement .* exceeds base index size .*" ../$r.err ||
+			grep -e "BUG: .* fsmonitor_dirty has more entries than the index .*" ../$r.err
+		) &&
+
+		cat ../$r.err
+	)
+'
+
+r="repo-6"
+test_expect_success FSMONITOR_DAEMON 'repo-6 : start with sparse-index and fsmonitor and then enable split-index' '
+	git init $r &&
+	test_when_finished "git -C \"$PWD/$r\" \
+		fsmonitor--daemon stop" &&
+	(
+		cd $r &&
+
+		git config core.fsmonitor true &&
+
+		mkdir initial &&
+		test_commit initial/initial &&
+
+		echo x >initial/x1 &&
+		echo x >initial/x2 &&
+		echo x >initial/x3 &&
+		echo x >initial/x4 &&
+		echo x >initial/x5 &&
+		echo x >initial/x6 &&
+		echo x >initial/x7 &&
+		echo x >initial/x8 &&
+		echo x >initial/x9 &&
+		echo x >initial/y1 &&
+		echo x >initial/y2 &&
+		echo x >initial/y3 &&
+		echo x >initial/y4 &&
+		echo x >initial/y5 &&
+		echo x >initial/y6 &&
+		echo x >initial/y7 &&
+		echo x >initial/y8 &&
+		echo x >initial/y9 &&
+		git add . &&
+		git commit -m "ballast" &&
+
+		mkdir one &&
+		test_commit one/one &&
+
+		mkdir two &&
+		test_commit two/two &&
+
+		mkdir three &&
+		test_commit three/three &&
+
+		git sparse-checkout init --cone --sparse-index &&
+		git sparse-checkout set one two &&
+
+		git config --list --show-origin &&
+
+
+		git config core.splitIndex true &&
+		git config splitIndex.maxPercentChange 0 &&
+
+		# Sparse then split correctly fails.
+
+		test_expect_code 128 git update-index --split-index >../$r.out 2>../$r.err &&
+		grep "fatal: cannot use split index with a sparse index" ../$r.err
+	)
+'
+
+test_done