I am so sorry that happened to you AppleDapple
My worst misfortune happened in City Folk.
My first one and worst of all was: I was on the ACC forums and had won a giveaway and the deliverer was in my town giving me my prize: a full Gracie furniture set (Can't remember which one), a ton of bells and a bunch of other stuff. While the delivery was going on, somebody I'd recently added came in and then the delivery person left. I started grabbing the furniture to put in my house, and she started taking the bells (this was before I knew you could end wi-fi) and then she left. Luckily, I got ahold of the delivery person on the forums and she brought me another set. And as for the thief, I deleted her fc and gave her a bad wi-fi rating.
My other one happened in NL: This one is nowhere near as bad as my first one. I was trading with someone and she was in a super big hurry, so I started dropping bells, thinking that I had it counted out right, picked up my items and she ended the session. When I got back to my town, I realized I'd dropped an extra 200k. I didn't really care about the extra 200k as far as wanting to get it back, I just went and played on the island for awhile to make some more bells. But what really told me what kinda trader she is, is that she never questioned it or brought it up. Personally, whenever anyone leaves anything extra or other than bells in my town, I always PM them and ask about it.