Humor, jokes, memes about TTRPGs
Seriously this opens up so much creativity for the DM.
"This person seems familiar to you, but you're not sure why."
"You think you've been here before, but you're not sure when."
"Being on a ship feels like home, but you have no memories of ever being on a ship."
"Upon seeing the ruins of the village you find yourself overcome with sorrow, you feel like you've lost something important but you don't know what."
Part of the adventure can be a quest to reunite them with their lost memories.
I walk up to the bartender and ask him about rumors.
Bartender: "You son of a bitch, I told you if you ever come back, you better be able to pay your tab!"
LoL make them pay for their lack of effort!
Love it
In other words, "My backstory is whatever you want it to be".
If you were the DM and this bothered you, the player just gave you powerful ammunition.
You could even have it so whenever the player entered a shop in his home town, the shopkeepers looked at him with disgust and refused to serve him. The DM wouldn't even have to necessarily come up with a reason. Just, that the player is extremely well known among the locals and they universally think he's absolutely disgusting and want nothing to do with him.
Yeah, i find this to be awesome, because i can now leverage the other player's back stories into WHY they lost their memory. Or use it as an inflection point to shove the players a bit.