diff --git a/CHANGELOG.md b/CHANGELOG.md
index b6c395c..45425ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,4 @@
+2026-08-25 00:26 UTC — Fix race condition in thumbnail candidate generation
 2026-08-25 00:20 UTC — Add autoplay, progress rings, duplicate-lesson detection, title-card thumbnails
 2026-08-24 23:22 UTC — Add Favorites, storage drill-down, shortcuts, and What's New
 2026-08-24 19:00 UTC — Fix video resize handle drifting off-screen
diff --git a/VERSION b/VERSION
index 64f37da..541a536 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2026-08-25 00:20 UTC — autoplay, progress rings, duplicate lessons, title-card thumbnails
+2026-08-25 00:26 UTC — fix race condition in thumbnail candidate generation
diff --git a/offlineu_core.py b/offlineu_core.py
index d4b5fa3..9ba7704 100644
--- a/offlineu_core.py
+++ b/offlineu_core.py
@@ -952,9 +952,14 @@ def _generate_course_thumbnail(course_dir: Path) -> Optional[str]:
     best_candidate = None
     best_size = -1
     candidate_paths = []
+    # Unique per call (not just per offset) so two concurrent requests for
+    # the same course - e.g. the Precompute background scan racing an
+    # ordinary page load that also misses the cache - never write or clean
+    # up each other's candidate files out from under one another.
+    run_id = uuid.uuid4().hex[:8]
     try:
         for i, offset in enumerate(candidate_offsets):
-            candidate_path = course_dir / f'.offlineu_thumbnail_candidate_{i}.jpg'
+            candidate_path = course_dir / f'.offlineu_thumbnail_candidate_{run_id}_{i}.jpg'
             candidate_paths.append(candidate_path)
             try:
                 result = subprocess.run(