While I do have a litany of things behind me, this one made me famous among my friends for years.
So, I had a buddy who would routinely sleep at my house and be stoned so hard he could barely use his face to make coherent words and paired this with drinking as often as possible. One evening he came home just as I'd returned from town with a bag of potato chips. Well, it was the end of the month so we didn't have food that you could easily grab and munch on, everything required motor skills. He staggered down to my room and asked me for the chips.
I told him flat out that the chips were extra special and had cost me 20 dollars. He jams his hand in his pocket, takes out his wallet, flips it open, stuffs a 20 in my hand, takes my bag of chips and disappears down the hallway. The next morning I'm woken by the sound of him shouting. "Some bastard has been stealing out of my wallet!" as he turns the couch over looking for the 20. He then decided it was one of his friends, jumped on the phone and started lambasting the guy. As such I declined to inform him that he'd purchased a 3 dollar bag of chips for an 85% markup. (Or thereabouts lol)
And no, I didn't return it. 14 year old me was a jerk.