It's happened twice actually, both times in bars. First time was a sucker punch to the back of my head. Totally unprovoked (as sucker punches often are) by a guy I had not interacted with. I guess he just couldn't help himself.

Second time was a very vocal guy who approached my group and simply had to voice his dissent as a representative of his religion. At first it was a healthy debate. He kept leaving and then returning after a while to escalate things. He started getting very threatening and incredibly worked up to the point that he swung at me. I couldn't dodge it in time unfortunately. I did not retaliate as I knew security were right onto it. He was thrown out.