Hitting villagers with the bug net, pushing them around, watching them fall into a pitfall, trapping them by digging holes around where they're standing...writing letters to tell them to leave...writing on the bulletin board about how much I hated the snowman I made because he kept complaining about how I made him look wrong...
...I used to be really mean to the villagers I didn't like when I was little. XD
For Wild World, using Action Replay and SAVING it onto my town (I don't even know what I was thinking... T-T) caused me to be annoyed by it later on and to reset my first ever town in the Animal Crossing series. That was my biggest mistake.
In both Wild World and City Folk, I regret time travelling. Once I realized it wasn't any fun anymore, I went back to playing day-by-day and enjoyed that much more. I have never time traveled on my New Leaf town, and never will.
On New Leaf, I haven't done anything too regrettable, thankfully.
I can't really think of any other major mistakes I've made, so I think I've had an overall good experience with the series.
I honestly had no idea about rare items and Bud kept on giving me items from his house, most of which are rare (i.e. beach table, yellow aloha shirt, surfboard) but I kept on giving them back to him. I even sold the beach table at retail for a meagre price as I am not a big fan of the beach theme and needed to make space for other things.
The biggest thing I can remember is not being able to prevent Kid Cat and Apollo from moving out in WW.
In NL nothing major so far...probably just the few times when my game was accidentally shut off without saving. I'm sure I'm going to regret being really mean to Miranda (I already feel bad about it) but I just want her gone, darn it.