My roommate last year took advantage of me. She would borrow some of my belongings, but never return them to me until I had to ask her or just take them back myself.
Once, she used up almost all of my apple sugar scrub, and the container was about half full when I let her borrow it. I'm sure she'd have used up the whole tub had I not asked her to return it to me.
She and her three close friends always asked me to drive them to the grocery store, and didn't always thank me or show any appreciation for the favors I was doing for them. I also drove them to Atlanta, GA from Gainesville, FL for an anime convention; my car was almost packed to the roof with all the stuff we bought, and on each drive they wanted to listen to their favorite music the whole way (we don't exactly share the same musical tastes). I nearly had a meltdown after I found out that I would have to sleep in extremely close quarters and share a room with about 11 or 12 other people. It was embarrassing.
I lent her a pair of sandals which my cousin in China bought for me, and even though she had her own shoes, she wore my sandals exclusively for a few weeks. My friends all wondered why she all of a sudden started wearing a new pair of sandals and stopped wearing her own shoes.
Oh yeah, and after I accidentally left my landline phone in the room after check out last year, she somehow lost it. She told me that she may have just tossed it out with some of her unwanted belongings. I know it's not her fault I forgot to pack the phone, but shouldn't common courtesy require that she at least keep it until I'm able to retrieve it?
So after all that, I'd safely say that I'd been taken advantage of quite a few times. I trusted her too much. Who was I to expect everyone to be trustworthy and fair?