Here's one from my early days. Not the most stupidest thing I've done, but mildly amusing.
Fresh PPL in hand, just about to take a friend up and show him what a great pilot I am. Starting up on the pad, doing the ATIS/radio gubbins. Tower come on and say "G-XXXX how do you read?". I have no idea what he's talking about (never heard that expression before on the radio). So I say "say again". He says again. I still have no idea what he's talking about. I had to ask him two further times before he said rather angrily "G-XXXX DO YOU READ ME LOUD AND CLEAR?", then the penny dropped. D'oh!
Needless to say my friend still reminds me about that to this day.