A couple of times recently I’ve tried to save the replay at the end of an online race, only to get a message that the download has failed and that I’ve been disconnected from the session. Aside from the irritation of getting disconnected and losing the replays, this leaves me with a “Replay Download Failed” popup just above the centre of the screen which does not time out. The only way I’ve found to clear it is to fire up another game, as it persists through shutdown/resume with instant-on enabled.
It’s good practice to do a system menu restart once in awhile to refresh the system, clean put the cobwebs. If you’re on your Xbox a lot, I’d do it at least once a week.