I was once offered this by KL @MUC. I arrived very early on a flight from PRG (completely separate booking) and just headed to check-in; I can't remember the exact times, but it was something like 4 or 5 hours to my booked flight. Unprompted, the lady said, "There's a flight just about to board - we can get you on that one, if you like!" No problem: BP printed (no checked luggage), through security (no queue - this was around 6:45 am), straight to the gate, front of the queue and boarded immediately! Nice. ^ (After all, the Crown Lounge at AMS is a nicer place to spend 4 or 5 hours than the AF 'lounge' at MUC is!)

OK, so I was on a (paid) Business Class (sorry, Europe Select, for those suffering nostalgia) ticket but it was a restricted "Z" fare - officially, no changes without fees. (In fact, she actually said something like, "I'll just check to see if your ticket is changeable," played around on her computer terminal, made some non-specific noise and carried on with the process! Maybe she remembered that Henry's Imperial House has Bavarian connexions!)

-- Henry