It was my first character and my first campaign. I decided to be a Rogue because it would be fun sneaking around. Everyone else had made characters seperately so no one really knew who had what equipment, which would lead to a lot of confusion down the road. In the second session, while the player for our very tanky dwarf barbarian stepped out to use the bathroom, I asked to DM if I could roll to steal his dagger off him. I got approval and rolled a Nat20. Yes! Now I had his dagger. I figured it would lead to some funny scene later when he goes to use it and finds out its not there, but wouldn't be detrimental like if I had stolen his main weapon, a Great Axe. Player came back, no one decided to let him in on my pickpocketting. Next session it comes to light that the Barbarian never had a dagger to begin with. In fact, no one in the party had a dagger starting out except me. The Paladin and Cleric had all Martial weapons, the Barbarian too due to his military background, and the Bard had opted for brass knuckles instead.
"So who's dagger did I steal?"
DM ended up deciding, since I did roll a Nat20 plus my Slight of Hand bonus, of which I was also proficient in; that I now had Schrodinger's Dagger. It may or may not be there and I would have to flip a coin to see if I could use it, then roll for attack. There were times where it was there but I would miss or hit and then times when I'd use my action discovering that it wasn't there.