For me it's simple. The Olympus Infinity Stylus. Man I hated that camera.
I bought one brand new from B&H Photo when I was living in New York. I wanted a pocketable camera to have with me 24/7 because in NYC there is always something photographic going on around you. The idea was, keep it in your pocket or backpack all the time, when you see something, quickly pull it out, slide the door open (which turned on the camera) and capture that exciting moment. What could possibly go wrong.
I'll tell you what, that damn camera failed to turn on when the door was slid open at least 1/2 the time. I missed so many shots because I couldn't get the damn camera to power on, even sliding the door back and forth, nothing would work. So I brought it back to B&H, and being B&H at that time, they were totally cool about it and gave me another brand new one. And it did the exact same stupid thing. After trying to get that one to work reliably for a couple of weeks, I finally smashed it into the pavement and then tossed it in the garbage.
Even now, 27 years later, I see one of those cameras and I just want to smash it to bits. A completely rational response I think. 😉
Best,
-Tim