I once ended up checking in at SFO @ 2pm for an 11pm flight. Long story - had changed from redeye to early afternoon flight in anticipation of weather in EWR canceling the redeye...only to have the early afternoon flight cancelled due to anticipated weather in EWR; I'd already scuttled my plans for the day, checked out of my hotel and was halfway to SFO when the cancellation came through. Had no problem checking my bag.