You just made me think. What was the last thing I bought impulsively? I generally don't like spending money.

A pocket calculator for €2.65. I have a phone. I don't need it. But now I have a calculator, always ready in pocket.

Stupidest one? Probably A4 sticker sheets of DVD labels. They were cheaper than regular A4 paper because nobody wanted that. I mean, you can carefully peel it in one piece, and if you tape it over, you got nice stickers. How many? Well, it's 500 sheets of 6 labels each, so 3,000 DVD labels. That was like €1.70.
It was one of those 99+% discount sales. Usually this label paper is around €0.60/sheet (A4), which would land this at €300.