OMG! I've had so many, but this one tops them all.
One of my friends had a big house party for their 18th birthday, she invited most of our year. Anyway, a few hours into the night everyone was pretty drunk, and my big crush at the time (he was straight), invited me to sit on his lap as a joke, so I accepted the offer "as a joke".

As the night progressed we got more and more drunk, and it was only me and him and a few other people around the table. I got so drunk that he looked at me, and I went in and kissed him - thinking he wanted me too.

He pulled away straight away and asked me what I was doing, I looked around and everyone was looking at me. I just got up and left. I apologized to him and to be fair, he was a real gentleman and never told anyone about it. Luckily, the other people with me at the time were close friends so I never got any backlash for it. But the pure though of it makes me want the ground to swallow me up, it was so stupid. Hormones can be crazy at times.
Oh, and being sick on a busy train is also pretty high up there. I'm such a mess sometimes.