diff --git a/backend/src/appointment/l10n.py b/backend/src/appointment/l10n.py
index 8cc39d8fa..1232c4710 100644
--- a/backend/src/appointment/l10n.py
+++ b/backend/src/appointment/l10n.py
@@ -1,10 +1,14 @@
 from typing import Union, Dict, Any
-from starlette_context import context
+
+from starlette_context import context, errors
 
 
 def l10n(msg_id: str, args: Union[Dict[str, Any], None] = None) -> str:
     """Helper function to automatically call fluent.format_value from context"""
-    if 'l10n' not in context:
+    try:
+        if 'l10n' not in context:
+            return msg_id
+    except errors.ContextDoesNotExistError:
         return msg_id
 
     return context['l10n'](msg_id, args)
diff --git a/backend/src/appointment/routes/auth.py b/backend/src/appointment/routes/auth.py
index d67513658..54de6fc3b 100644
--- a/backend/src/appointment/routes/auth.py
+++ b/backend/src/appointment/routes/auth.py
@@ -7,6 +7,7 @@
 import argon2.exceptions
 from fastapi.security import OAuth2PasswordRequestForm
 from jose import jwt
+from sentry_sdk import capture_exception
 from sqlalchemy.orm import Session
 
 from fastapi import APIRouter, Depends, HTTPException, Request
@@ -126,6 +127,18 @@ def fxa_callback(
     elif not subscriber:
         subscriber = fxa_subscriber
 
+    fxa_connections = repo.external_connection.get_by_type(db, subscriber.id, ExternalConnectionType.fxa)
+
+    # If we have fxa_connections, ensure the incoming one matches our known one.
+    # This shouldn't occur, but it's a safety check in-case we missed a webhook push.
+    if any([profile['uid'] != ec.type_id for ec in fxa_connections]):
+        # Ensure sentry captures the error too!
+        if os.getenv('SENTRY_DSN') != '':
+            e = Exception("Invalid Credentials, incoming profile uid does not match existing profile uid")
+            capture_exception(e)
+
+        raise HTTPException(403, l10n('invalid-credentials'))
+
     external_connection_schema = schemas.ExternalConnection(
         name=profile['email'],
         type=ExternalConnectionType.fxa,
diff --git a/backend/test/integration/test_auth.py b/backend/test/integration/test_auth.py
index 1bed6976f..73492b1d8 100644
--- a/backend/test/integration/test_auth.py
+++ b/backend/test/integration/test_auth.py
@@ -1,6 +1,7 @@
 import os
 
-from defines import FXA_CLIENT_PATCH
+from appointment.l10n import l10n
+from defines import FXA_CLIENT_PATCH, TEST_USER_ID
 from appointment.database import repo, models
 
 
@@ -89,3 +90,34 @@ def test_fxa_callback(self, with_db, with_client, monkeypatch):
             fxa = subscriber.get_external_connection(models.ExternalConnectionType.fxa)
             assert fxa
             assert fxa.type_id == FXA_CLIENT_PATCH.get('external_connection_type_id')
+
+    def test_fxa_callback_with_mismatch_uid(self, with_db, with_client, monkeypatch, make_external_connections, make_basic_subscriber, with_l10n):
+        """Test that our fxa callback will throw an invalid-credentials error if the incoming fxa uid doesn't match any existing ones."""
+        os.environ['AUTH_SCHEME'] = 'fxa'
+
+        state = 'a1234'
+
+        subscriber = make_basic_subscriber(email=FXA_CLIENT_PATCH.get('subscriber_email'))
+
+        mismatch_uid = f"{FXA_CLIENT_PATCH.get('external_connection_type_id')}-not-actually"
+        make_external_connections(subscriber.id, type=models.ExternalConnectionType.fxa, type_id=mismatch_uid)
+
+        monkeypatch.setattr('starlette.requests.HTTPConnection.session', {
+            'fxa_state': state,
+            'fxa_user_email': FXA_CLIENT_PATCH.get('subscriber_email'),
+            'fxa_user_timezone': 'America/Vancouver'
+        })
+
+        response = with_client.get(
+            "/fxa",
+            params={
+                'code': FXA_CLIENT_PATCH.get('credentials_code'),
+                'state': state
+            },
+            follow_redirects=False
+        )
+
+        # This should error out as a 403
+        assert response.status_code == 403, response.text
+        # This will just key match due to the lack of context.
+        assert response.json().get('detail') == l10n('invalid-credentials')