Like someone else said, never had "bad" sex...but I do remember one girl in particular that was extremely disappointing. Back in my teen years there was this girl I fucked a bunch of times, with the hopes over time she would get over her inhibitions. She was a lot of fun in every other aspect, partied her ass off and was amazing eye candy under my arm.
But SO fuckin timid in bed. Would only do it missionary, because she said any other position would "make her feel like a slut". If she'd suck me she'd constantly remind me..."you better not cum". When I went down on her, she wouldn't let me kiss her hours after even when I showered and brushed my teeth because.."you've been down there and that is gross". Best sex we ever had together was while tripping on acid, but that isn't much of a commentary.
What a fuckin waste for such an amazing bod, and drop dead gorgeous looks.
I ended up fucking her little sister twice, and while she was no porno star....she ended up being a bit naughtier and better.