When I'm in a cafeteria I will deliberately choose a table where nobody is sitting so that I can eat quietly and happily. Myself, my food and my thoughts....
Then somebody joins me, and another, and another and with each addition I become more uncomfortable and anxious!

The worst is when someone plonks themself down right across from you so that it is impossible to look up without looking into their face, OH, THE TORTURE OF IT ALL!
Oh man. The worst ever? You asked for it. I dated an extrovert ...
I was dating this girl who lived in one of those tall apartment buildings in New York. She had a thing for making out or having sex in somewhat public places. In her building, we used to go make out in the very top of the building's stairwell. That was actually a pretty hidden "public place" as public places go. One time, someone heard us and stuck his head around the corner to take a peek. He sorta grinned and left. Not really very awkward, from my point of view.
Then, she got it in her head to try having sex in the elevator late at night. I thought this was a very bad idea. But, I wasn't really thinking with my brain.
So, there we are in the elevator. She leaned against the wall, and I kneeled on the floor and went down on her. Then, she leaned me against the wall, and returned the favor. She pushed me down on the dirty floor of the elevator, and was straddling me when the elevator started moving.
She jumped off me, and we both got dressed with amazing speed. However, the elevator must have reeked of sex. Also, I was only wearing pajamas and I was sporting a massive hard-on.
The doors opened. A middle-aged woman who was dressed like she was going to work got in. My girlfriend and I stood at one end of the elevator, the middle-aged woman at the other. Time passed very, very, very slowly. I was utterly horrified by the experience. It went on for almost a minute.
Ding.
She walked out without saying a word. The second the doors closed, my girlfriend passionately tore off my pants, threw me to the floor and rode me hard and fast like getting caught was the hottest thing ever. I helped go through the motions, still feeling pretty traumatized.
Hmm ... thinking if this was the MOST awkward situation I've ever been in, I'm not totally sure but this sort of situation would be my least favorite.