diff --git a/cloud/blockstore/libs/diagnostics/critical_events.h b/cloud/blockstore/libs/diagnostics/critical_events.h
index ccaa9cb3454..ca6a506c7f2 100644
--- a/cloud/blockstore/libs/diagnostics/critical_events.h
+++ b/cloud/blockstore/libs/diagnostics/critical_events.h
@@ -64,6 +64,7 @@ namespace NCloud::NBlockStore {
     xxx(DiskRegistryCleanupAgentConfigError)                                   \
     xxx(DiskRegistryOccupiedDeviceConfigurationHasChanged)                     \
     xxx(MirroredDiskChecksumMismatchUponRead)                                  \
+    xxx(DiskRegistryWrongMigratedDeviceOwnership)                              \
 // BLOCKSTORE_CRITICAL_EVENTS
 
 #define BLOCKSTORE_IMPOSSIBLE_EVENTS(xxx)                                      \
diff --git a/cloud/blockstore/libs/storage/disk_registry/disk_registry_state.cpp b/cloud/blockstore/libs/storage/disk_registry/disk_registry_state.cpp
index 1591631abf5..0ee39952938 100644
--- a/cloud/blockstore/libs/storage/disk_registry/disk_registry_state.cpp
+++ b/cloud/blockstore/libs/storage/disk_registry/disk_registry_state.cpp
@@ -5032,18 +5032,10 @@ void TDiskRegistryState::ApplyAgentStateChange(
                 continue;
             }
 
-            bool found = false;
-            for (const auto& d: disk.Devices) {
-                if (d == deviceId) {
-                    found = true;
-                    break;
-                }
-            }
-
-            if (!found) {
-                // this device is allocated for this disk but it is not among
-                // this disk's active device set
-                // NBS-3726
+            if (Find(disk.Devices, deviceId) == disk.Devices.end()) {
+                ReportDiskRegistryWrongMigratedDeviceOwnership(
+                    TStringBuilder() << "ApplyAgentStateChange: device "
+                                     << deviceId << " not found");
                 continue;
             }
 
@@ -6145,9 +6137,10 @@ NProto::TError TDiskRegistryState::FinishDeviceMigration(
     auto devIt = Find(disk.Devices, sourceId);
 
     if (devIt == disk.Devices.end()) {
-        // temporarily commented out, see NBS-3726
-        // return MakeError(E_INVALID_STATE, TStringBuilder() <<
-        //     "device " << sourceId.Quote() << " not found");
+        auto message = ReportDiskRegistryWrongMigratedDeviceOwnership(
+            TStringBuilder() << "FinishDeviceMigration: device "
+                             << sourceId.Quote() << " not found");
+        return MakeError(E_INVALID_STATE, std::move(message));
     }
 
     if (auto it = disk.MigrationTarget2Source.find(targetId);
@@ -6174,27 +6167,16 @@ NProto::TError TDiskRegistryState::FinishDeviceMigration(
     }
 
     const ui64 seqNo = AddReallocateRequest(db, diskId);
-    // this condition is needed because of NBS-3726
-    if (devIt != disk.Devices.end()) {
-        *devIt = targetId;
-        disk.FinishedMigrations.push_back({
-            .DeviceId = sourceId,
-            .SeqNo = seqNo
-        });
+    *devIt = targetId;
+    disk.FinishedMigrations.push_back({.DeviceId = sourceId, .SeqNo = seqNo});
 
-        if (disk.MasterDiskId) {
-            const bool replaced = ReplicaTable.ReplaceDevice(
-                disk.MasterDiskId,
-                sourceId,
-                targetId);
-            // targetId is actually fully initialized after migration
-            ReplicaTable.MarkReplacementDevice(
-                disk.MasterDiskId,
-                targetId,
-                false);
-
-            Y_DEBUG_ABORT_UNLESS(replaced);
-        }
+    if (disk.MasterDiskId) {
+        const bool replaced =
+            ReplicaTable.ReplaceDevice(disk.MasterDiskId, sourceId, targetId);
+        // targetId is actually fully initialized after migration
+        ReplicaTable.MarkReplacementDevice(disk.MasterDiskId, targetId, false);
+
+        Y_DEBUG_ABORT_UNLESS(replaced);
     }
 
     *diskStateUpdated = TryUpdateDiskState(db, diskId, disk, timestamp);
diff --git a/cloud/blockstore/libs/storage/disk_registry/disk_registry_state_ut_migration.cpp b/cloud/blockstore/libs/storage/disk_registry/disk_registry_state_ut_migration.cpp
index 1939f31d670..619b180e294 100644
--- a/cloud/blockstore/libs/storage/disk_registry/disk_registry_state_ut_migration.cpp
+++ b/cloud/blockstore/libs/storage/disk_registry/disk_registry_state_ut_migration.cpp
@@ -314,6 +314,14 @@ Y_UNIT_TEST_SUITE(TDiskRegistryStateMigrationTest)
             UNIT_ASSERT_VALUES_EQUAL(0, affectedDisks.size());
         });
 
+        NMonitoring::TDynamicCountersPtr counters =
+            new NMonitoring::TDynamicCounters();
+        InitCriticalEventsCounter(counters);
+        auto configCounter = counters->GetCounter(
+            "AppCriticalEvents/DiskRegistryWrongMigratedDeviceOwnership",
+            true);
+        UNIT_ASSERT_VALUES_EQUAL(0, configCounter->Val());
+
         executor.WriteTx([&] (TDiskRegistryDatabase db) mutable {
             auto affectedDisks = ChangeAgentState(
                 state,
@@ -331,6 +339,9 @@ Y_UNIT_TEST_SUITE(TDiskRegistryStateMigrationTest)
         }
 
         UNIT_ASSERT(state.IsMigrationListEmpty());
+        // Now bug is fixed, but, if it reproduce in future, we must report
+        // event.
+        UNIT_ASSERT_VALUES_EQUAL(1, configCounter->Val());
     }
 
     Y_UNIT_TEST(ShouldEraseMigrationsForDeletedDisk)