I once had a friend (who knew I wasn't interested in him) stay over--we had a discussion before falling asleep that nothing was going to happen & the only reason he was in my bed is because I didn't have anywhere else for him to sleep. He ended up waking me up in the middle of the night trying to have sex with me. In my half-awake state, I went along with it at first, but stopped as soon as I realized what was going on. It really pissed me off & I felt completely violated!
However, the guy I'm with now knows I'm okay with it & he's done it a few times & I loved it.