Not a gay bar experience per se, but I was once hit on by a gay guy at a straight bar. In hindsight it seems hard to believe, but it did happen. This was when I was in college, and apparently the guy was recently out and was attempting to hone his gaydar. Somehow I gave him the impression that I was signalling him and he started getting pretty friendly. It took me quite a while to realize what he was doing, since I just never expect that kind of attention from another guy.
After about 10 minutes, I said something that clued him in to his mistake, and his reaction made me realize what was going on. (I guess I was slow on the uptake what with having a few beers in me and all.) He was pretty embarrassed, and I told him he should be careful about doing that, since a lot of guys would not be nearly as understanding as I was.
I felt kinda weird about it afterward, but later I decided I should take it as a compliment.