A slip of the pen/tongue
Definition 1 of 3
Pro Tip 1/3Accidental Disclosure
Used when someone accidentally reveals information that was supposed to be confidential or private. 
He mentioned the surprise party; it was a slip of the tongue.
Pro Tip 2/3Writing Corrections
Refers to minor mistakes while writing, often amended through editing or proofreading. 
Erroneously writing '2012' instead of '2021' was a slip of the pen.
Pro Tip 3/3No Harm Intended
Suggests the mistake was unintentional and the speaker or writer did not aim to offend or confuse. 
It was a slip of the tongue when I used your old title; I meant no disrespect.