When has flirting "gone too far" for you?

So a little innocent flirting is usually a good thing, mainly when both parties are interested. But when does the “flirting” go too far? When do you start to feel uncomfortable with the situation? What do you do when the flirt-er goes too far?

I ask because I just had an inebriated “friend” put his arm around me a couple nights ago. I realized this was my “gone to far” moment. I just find that an arm around my shoulder is a very personal thing, and almost is an act of “claiming” me as his. Maybe I’m overreacting..