What happens between check in and boarding?

[TightSlot]

Don't forget that on flights to the U.S. there are the security requirements for pax APIS info to be sent to the various authorities in advance of departure, requiring an absolute cut-off time.

FULL SERVICE airlines won't generally be petty enough to deny you check-in and boarding if you're a little late for a valid reason.

Sadly, not so. The point about valid reasons for being late is that everybody has one - nobody ever arrives at check-in claiming that they are late for no good reason whatsoever.