Ferric chloride was used to etch nickels to reduce the fields make the lettering slightly more raised to be able to tell mint mark. but if it was FC, this one soaked in it for a long period of time. Perhaps the person left it for summer vacation. Jim
To me, this one doesn't look like acid damage -- it looks like it was partially melted. Can you weigh it? What does the edge look like?
Regardless if it was acid or something else, your coin couldn't have left the mint in that cindition, so it's not an error.
maybe a burn barrel then cleaned up? looks like high heat did damage, maybe even a commercial furnace.
Whatever caused this I wasn’t there but I can say it’s damaged as it didn’t leave the mint looking like that.