It's tough to choose just one.
First, an honorable mention. My 4" Smith & Wesson M29 disappointed me in that it didn't stand up to continued use. In short order, it went from new to having cylinder end shake, lots of rattles, and the little plastic insert in the front sight departed for parts unknown. I shot it from rollover prone with my left forearm at about 90 degrees to the boreline and it spat enough bullet shavings out of the cylinder gap to leave my arm covered in blood after 20 rounds or so. Of course, this was back in the late 1970s when Smith wasn't known for ultimate durability and I was shooting full power, heavy bullet loads in high volume while competing in silhouette matches. But still...
But the winner is my Kahr K9. What. A. Piece. Of. Crap.
I have an early production piece. I accepted the jiggly front sight as the smooth trigger and nice function gave me reasonable confidence. Then, about 500 rounds in, the recoil spring jumped off the guide rod during recoil, stopping the slide about a half-inch out of battery. Emails to the factory were returned with instructions about how to retract the slide, remove the slide stop, then pull the trigger as the slide went forward to disassemble the gun.
Any pistol designed such that this can happen is fatally flawed. But did I learn my lesson? Oh, no. I reassembled the gun and went back to the range. Insert loaded mag, hit the slide stop to release the slide and it stops, again. This time, there's a round in the chamber and I can't eject it. I go home, attempt the previous repair, but can't get the thing disassembled, so I put the slide stop back in, intending to forget it for the night. Then I tried one last time to move the slide forward by pulling the trigger as I pointed the loaded gun in a safe direction.
If you know Kahrs, you can guess what happened. I had put the slide stop back in out of alignment. The slide went forward to a point about a quarter-inch out of battery where it is now locked tight. My research has thus far told me that I must send the pistol back to Kahr for disassembly and repair. And there's still a round in the chamber.
Any pistol that can be reassembled in a way that renders it completely inoperative and impossible to fix by simply disassembling and reassembling correctly possesses an unforgivable design flaw.
That round in the chamber? It's been there for years. It may just stay there until my heirs deal with it.
If anybody knows any tricks for dealing with this (or a gunsmith near Houston who can and also won't laugh at me), I'd be appreciative.