If I'm about to upload a comic a couple minutes before midnight, I choose tomorrows date, then I either get stuck writing my comments for too long or something distracts me, I come back and click upload and a php, or something, error will display making me think it hasn't worked.
If I check my comic then I'll see it has worked but this is still an annoying message to see. Someone unacquainted with this might even try updating again only to end up with two of the same updates.