I don't understand most people that have "shameful purchases" given you know, I actually do my research before buying something.
But if I ABSOLUTELY have to pick something, it would be Hydrophobia. It's a AA game from the early 2010's. It's main gimmick was amazing (for the time) dynamic water physics. Visually it was perfunctory for it's day, platforming, puzzles and story was mostly a joke. It's a 6/10 game at best. But I played it pirated originally, liked it sufficiently for the tech demo ambition that it had and purchased it a decade later for 2.5€ or something, maybe 0.5€?
Yeah, that's my story.