The most common and most likely answer to this crossword clue is the 7 letter word BOO-BOO
I've seen this crossword clue in The Guardian
B
O
O
-
B
O
O
Buddy explains!

The straight part
We know that "error/error" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case error/error = boo-boo.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BOO-BOO mean?
BOO-BOO noun- A mistake or error.
- (by or to young children) A minor injury, such as a cut or a bruise.
- (by or to young children) Feces.
- (by or to young children) To defecate.