Two times I can remember. For some reason, both involve the idea of threesomes.
One time, a friend's girlfriend started heavily suggesting a threesome between her, me and a girl I was seeing at the time. We were all together at the same gathering, including my friend. He seemed even more uncomfortable with it than I was, but apparently, they had an "open relationship" (which only she benefited from, from what he told me).
The second time, I was sitting in a couch with another friend and we were talking. This friend had a crush on a girl for the longest time, so he started flirting with her and I decided I'd wing for him.
At some point, for some reason, he was tickling her and she laid down on the couch in a way that she was over both our laps. I think she wanted to sleep with the two of us at the same time. He starts touching her more intimately and she starts lowkey moaning in my lap, making strong eye contact with the two of us.
God, that made me so uncomfortable to the core. It irked me so bad I had to resist the urge to shove her away from me. I don't know what about that experience bothered me so much, but it really did.