I have a few, a couple are funny, one was just very akward and I felt bad.

First, in high school, I was like 16 so my g/f and I were screwing like 5 times a day, at one point she turned around when we were going doggy style, the matress was hanging off the bed so when she put her weight on it she slipped off and landed on my chair face first, she was ok so we just laughed.

Next when I was with the only virgin I have slept with, I said I wanted to do it doggy style, and she gets on all 4s with her legs close together, her butt facing down down and her back up in the air, and just looked around the room like she had no idea what was going on, I laughed a bit, pushed her back down, turned her butt up and spread her legs, I was like "There this is how you are supposed to kneel".

Last one was sad, my g/f at the time was giving me a blowjob, I was hard and horny but for some reason she couldn't get me off, I would get close but then nothing. She wouldn't give up for like half an hour, finally she stopped and rolled over all sad, was very akward, I had no idea what to say and felt really bad.