slip of the pen; mistake in writing
Refers to the act or result of making a writing error, such as a misspelling or wrong character.
See also: 書き損じ
There was a slip of the pen in the letter, so I rewrote it.
something ruined by a writing mistake
Often refers to a physical item like a postcard or form that is spoiled because of a writing error.
See also: 書き損じ
I ended up with several postcards ruined by writing mistakes.