We all have at least a few....
RIA 1911 5" .45 Auto. I don't actually regret this one so much - it's ugly, but 100% functional for range purposes. I just don't take much pride in owning it and its utility is worth way more than the $200 I'd make in resale at this point.
Tanfoglio TZ-75. Beautifully built and finished, refused to function. Flipped at a heavy loss.
Gen 3 Glock 21 I didn't hate this one actually, but my hands were far too small and the Gen 3 texture far too slipper for this combo to ever work out. I haven't tried a Gen 4, but I imagine I'd probably like that one better.
Gen 4 Glock 23. Refused to function reliably even with 180gr Federal HST. Flipped at a heavy loss.
Gen 3 Glock 19. Because it made me buy so goddamn many other glocks...