This is totally evil, but happens all the time. I once arrived earlier than previously planned at an intermediate stop where a change of carrier was involved. I asked if there was a seat available on the earlier flight than the one I was booked on, and about to depart ? Yes, Oh ! good, I have my later flight boarding pass, no luggage, can I go now ? Yes, welcome, pay me $50. Why ? Administrative fee. What ? I agree that if I had contacted you before arrival you might have had some massive amount of admin. to complete, but here I am, there is the gate about to close, I have no luggage, you don't provide catering ( a local puddle-jumper ) and you say that there is a seat available that is "free" seating, no seat numbers have to be issued, I just walk on. No. OK, if I fill one of your empty seats now, you may be able to re-sell my previous one for the afternoon flight. No, only if you pay us $50. Get stuffed, and I sat in front of the desk for the rest of the time reading my book and occasionally laughing at them. But of course they didn't care.
About 20 years ago I went from Stockholm to Brussels for the day for a meeting. My outbound flight was delayed and I actually got to the meeting just as it ended. I was booked to return with SAS at 17.00 but had hoped to maybe get on to a 15.00 Sabena flight if the meeting was short. I got to the airport about 15.10 and went to check in for the 17.00 where I was asked if I would be prepared to take an earlier flight of they gave me $100 USD cash. I pointed out that there wasn't an earlier flight and they told me that the 15.00 Sabena flight was delayed until 16.15. So I took my $100 and went to check in at about 15.20 for the delayed 15.00 flight where I was given a voucher to buy some food or drink due to the delay.