There have been more than a few things I could write about...
One in particular was at uni - went shopping with flatmate, bought a very distinctive tshirt, put it in a drawer and went to get it a few days later - not there. Was a bit upset but assumed I'd put it somewhere else and forgotten about it. It was just a top and wasn't expensive so I got over it and assumed it would turn up eventually.
Some weeks later, I was looking for something in one of my flatmate's drawers in her room (she was out and had asked me to bring her a change of clothes so we could go out after her lecture) and found the tshirt folded up inside another top - still with the tag on, and definitely mine as it was in the sale so had a distinctive hand-written markdown. I was a bit stunned - hadn't crossed my mind that she would do that at all. She'd actually said she wasn't keen on it in the shop when I bought it!
I never said anything and neither did she (assuming she noticed it was no longer there, and that she realised what had happened!) but that opened my eyes a lot and I started noticing a few small things that I'd previously let slide, like using up food/drink I'd bought (not for communal use), or borrowing makeup/stationery/CDs and not returning them. I never called her out on it because I was scared of losing a friend/flatmate and didn't like confrontation, but was not terribly surprised when, after we moved out at the end of uni, she didn't pay her fairly sizable share of a bill which I had to cover, and totally blanked me on all calls/emails.
I was (and still am) actually more upset about the loss of what I had thought was a good friendship than the money/items. It was obviously quite one-sided! I just view it now as the literal price of having had some really good times with her (Rent-A-Friend, anyone?!) and try to not get upset about my naivety.