Limbo of the Lost
Plenty of adventure games torture players with incomprehensible puzzles and endless locked doors, but Limbo of the Lost did that while also committing rampant plagiarism. Constructed using recycled environments from games like Oblivion and Thief: Deadly Shadows, even the interface was cobbled together from bits of Diablo II and Black & White. It also pinched dialog from movies like Spawn and the Pirates of the Caribbean series. Once the endless list of copyright violations were discovered it was removed from sale, which would be no great loss if it weren't for the uncharacteristically wonderful ending: a full-on musical number that has to be seen to be believed.