A quick observation of how a reservation gets out of sync. Wednesday night I called the CP desk to move a return flight from Fri to early Thurs. The CP desk agent advised that since I was calling after midnight and some overnight system batch process had already run, the res would be out of sync and it was likely I'd need this manually fixed at the airport or by calling back in the morning. Sure enough, I couldn't check in online or by kiosk and after about three minutes of feverish typing at her desk computer, I was checked in.

I think this must have something with how reservations are prepared in batch to be moved to airport control. Just a guess. I doubt it applies to API being available, as it was already in there for my outbound legs.