I currently have a Star Alliance award booked as SFO-(UA)-IAD (22 hour non-stopover connection) - (UA) - FRA - (LH) - BCN (stopover for several days) - (JK) - MAD (end destination for several days) - (LH) - FRA - (LH) - SFO. The SFO-IAD flight arrives at 7pm, and the IAD-FRA flight leaves at 5pm the next day, giving me a night to visit friends in IAD without making it a second >24 stopover, which would violate award booking rules.

My question is: what if I were to standby for an earlier SFO-IAD flight (via the united.com online check-in) that would result in me having more than 24 hours in IAD? According to the letter of the law, this would of course result in me having a second >24 stopover and violating the rules. My question is about what happens in practice. In my experience, when standing by for an earlier flight and clearing at the gate, the PNR rarely actually gets updated with the earlier flight you actually took--your originally booked flight usually remains in the record, even though you have a boarding pass for the earlier one. And we know that the airport standby process usually works independently of whatever the original fare rules are--if there's space on an earlier flight, you get it, even if your original fare was only good for a flight later in the day, etc.

So my question is, in practice, would I be able to standby and take an earlier SFO-IAD flight? And if I am allowed to do that without anyone noticing, will I have any problems when I check in for IAD-FRA the next day--is anyone going to notice that I had been in IAD more than 24 hours?

Speculation welcome, but I would be especially interested if anyone has any first hand experience trying to do something like this.