Goof

ERR

Mistakes often start with this three-letter word found in "error."

"Goof" is a casual word for a mistake, which directly hints at the answer "ERR"—a short, formal term meaning to make an error. The clue plays on the idea of informal and formal language, where "goof" (slang) and "err" (more proper) essentially mean the same thing. It’s a straightforward definition with no hidden wordplay, just a simple synonym swap.