Just thinking about trivial things I should just have spoken out about/said no to when I was younger (20s and 30s). I'm lucky, of course, that there's nothing more major, but a few things still niggle. Things like....
When we went to stay with ILs, we'd stay at DH's parents' house. SIL (who lives nearby) would regularly come round with her toddlers (we also had toddlers at the time), put them to bed in the double bed we stayed in, already with our toddlers, and then leave them to sleep in there with me (DH would have to sleep on the sofa). Never once asked. So I'd just have to sleep in a bed with 4 toddlers, and she and her DH would go home and have a lovely "night off". Why didn't I say "Er, SIL..."?
As a postgrad, I had a housemate who liked to put half a load of washing in the machine, and leave it there (all day/for two days) while she gradually filled it up to wash. So no one could use the washing machine for days, sometimes. I took her washing out once so I could use it and she threw a massive tantrum. I apologised. Why didn't I tell her she was being a nob?
A friend used to ask me to shop for them sometimes as she was very busy with little kids and didn't drive. So I'd do her shopping, which I was happy to do. And sometimes, she wouldn't pay me for the shopping. Must have added up over the years. Why didn't I just ask/remind her??
Does everyone have these niggles?! Why do I still think about them, years later?!