A bad item in a game? Well, games are meant to have challenges, so it's subjective, but let's consider a frustrating item:
Fangame: Downloading a fangame and discovering it's a Kaizo game can be really frustrating. This has happened to me with Mario hacks.
Super Mario Bros 2: In the final level, the mask that always helped you progress suddenly turns against you! That was a low blow from Nintendo & Fuji Television* to include that in the game. I remember when I reached that part, I had only one life left, did everything to avoid dying, and ended up dying in the most unfair way.
*Explanation: SMB2 is actually Yume Kōjō: Doki Doki Panic (Developed by Nintendo in cooperation with Fuji Television) . Nintendo made some changes to the game, replacing the original protagonists with Mario series characters and making a few alterations. For example, the magic potion was originally a genie lamp in Doki Doki Panic, serving the same purpose – it's just a graphical difference.