right next to me in a 20/20 otb game there was this little kid of about 8 jumping on his chair and singing while playing the game...
OTB: Weird Things Your Opponents Did While Playing

I played a game with an older guy who like to munch on altoids... we were both 1600ish.. and he got a better position.. I was a move or two away from resigning.. so I just started playing relaxed and loose since I figured I was lost anyway.. and he just started making 2nd best/3rd best moves one after another.. and my position started slowly improving.. and as it did he started popping altoids faster and faster.. one after another.. and sweating more and more... as this went on, sure enough he walked his position right off a cliff and he finished his whole tin of altoids and was sweating so much I thought he was going to have a heart attack.. I almost felt bad winning that game.

Played a really weird person who made a funny movement with his head and made a throaty "kha" sound every time he punched down his clock.

There's been 2 players with strange ticks I had encountered.
One of them was a relatively old guy from Czechia who was humming and eventually even singing throughout the tournament.
The other one was a boy, age 9 or so, who "J'adoube"d almost all of his pieces like every other move. And instead of just doing it, he was whispering "J'adoube" for every single piece.

FUNNIEST THING I EVER EXPERIENCED IN CHESS: in the marshall chess club, and im playing a tournament. I am playing a strong child at the age of 10 and i am like 14 and his parents are about 20 ft away looking at him. i was playing him and i had a clear winning position. after about 3 minutes he put his hands on his head and whispered in a really raspy voice " I HATE YOU" and i started cracking up. im not sure if he was aware that i could hear him.
another funny thing is that i was once playing a player with autism and he kept on muttering under his breath " WE DON'T EVEN SPEAK CHINESE AAAAAAH". we drew.

My opponent repeatedly put his finger into his nose holes or ear and then licked that finger. When I won he ran to his mother and cried (he was 8)
(his behaviour during the game was rather disgusting for me...)

this weird irritating guy who i was playing in blitz pressed the clock twice after every move just to make sure it was pressed. it was terribly annoying as sometimes when it was my turn to press the clock he was still pressing it.

My library hosts a little kids tournament every year. In one game, both players argued over who could be the loudest without technically breaking the rules. They slammed the pieces, yelled check as loud as possible, j'adoubed for no reason, and were generally boisterous in any way they could be. The TD had a fit and kicked them out.

I was playing a game at a club meeting and the guy I was playing had his WHOLE family with him, they were at my board and commenting to each other, and snickering.There were about 7 people all staring it was quite unnerving and the guy was saying to them "I know! I know!" It quite an intense game (well for me! they were all staring!) i lost by one point from a draw :(
Happy ending though 2 weeks later I was in a tournament and with no one commenting and snickering I beat him bad :)

My library hosts a little kids tournament every year. In one game, both players argued over who could be the loudest without technically breaking the rules. They slammed the pieces, yelled check as loud as possible, j'adoubed for no reason, and were generally boisterous in any way they could be. The TD had a fit and kicked them out.
Lmao, I bet he had a fit. Surprised he didn't frisbee a chess board in their general direction first.

this weird irritating guy who i was playing in blitz pressed the clock twice after every move just to make sure it was pressed. it was terribly annoying as sometimes when it was my turn to press the clock he was still pressing it.
I always press the clock twice. It's just a small habit. And, one child at a tournament found it necessary to loudly exclaim that there was 3 move repitition, and argued with the arbiters.

The weirdest would be the opponent who constantly kept looking at his watch, all the while shuffling and sqwirming in his seat. He wasn't at all subtle about the watch thing either. I quit OTB after that.
My usual strategy when opponents did this sort of thing was to play extra slow, but this guy was full on. I'd end up moving just to stop him from looking at his watch.(The game was a draw.)
While this topic is just begging for the troll post or three, I thought it would be fun and interesting to post anyway.
Back during my WSCA days in High School I once played a strong 13ish year old female who was drawing a unicorn on a pocket notepad. I won that game but will never forget the "wtf is she doing" reaction I had for most of the game.
What were some of your more memorable experiences with players who, while minding their own business, still managed to make you cock an eyebrow?