I'm on a flight tomorrow (LAX-IAH 12:30PM). When I try to check in I get the message: "Unable to Complete Check-in. Check-in has been restricted to the gate. Airport check-in is required."

I called and my ticket is fine, nothing unusual going on. They had no idea why. Every time before this I've gotten this message it's been because my ticket was messed up in some way.

I can see on the upgrade standby list that only one person is checked into the flight and no one is on the upgrade list, which is highly unusual. So I'm guessing that means that nobody (essentially) has checked in for this flight and it's not just me.

So at this point I'm just curious -- what kinds of things cause a flight to be restricted to gate check in?