Originally Posted by fastair

NY-
There is no way to issue a boarding pass for a flight once the flight has been put in "post departed" status in the computer (usually after push back or just after take off.)

To be able to issue one at the destination means that the upline station never "closed out the flight" properly or sending the appropriate passenger data to HQ. This is rare, I would say a few percent of flights at most. Your ability to do this with 100% success on numerous occasions has staggering probabilities against you, but still within the realm of possibilities.