Someone sent you money by mistake? Send it back. “Their typo, my win” is a lousy personal motto.

An error that lands in your favor is not a gift, and treating it like one requires a small act of self-deception you have to keep performing. On the other side of it is a person who is now short, probably checking their balance, possibly panicking. The money did not appear from nowhere. Moments like this are quiet tests precisely because nobody is coming to enforce anything.

Send it back the same day, before the story you tell yourself gets more comfortable. A short message saying you think this came to you by accident handles it. The delay is the hard part, not the sending.