I once used a lot of Avios to book me and my US-based business partner to fly back from LAX in F. His seat was behind mine (old F). He decided on the day he had to stay in the US as an important issue came up. We couldn't cancel/change his flight.

I boarded, given my welcome champagne and settled in with a spare seat was behind where he would have sat. A young-ish girl, in her 20s, gets brought up to the seat and the crew member said "this is quite a big upgrade, hope you enjoy!". Within 2 minutes the young girl was making a call and you could hear her banging on loudly how she'd been upgraded from WTP to F, for free. I ended up complaining to crew that I'd paid for the seat and it was a bit unfair me having to listen to that. Rather than them saying "we understand, we'll have a quiet word", I was told "sir, if someone decides to no-show the seat booking is voided and can be resold or someone upgraded as in this scenario. Effectively you don't have that seat booking anymore!" which was more than a little stern.

Affected my entire flight, as I rarely fly F, even though the crew member was right...!