By mistake
The cheque was sent in error.
Mistaken
You are in error in your conclusions.
Any man is liable to err, only a fool persists in error. Cicero
Any man worth his salt will stick up for what he believes right, but it takes a slightly better man to acknowledge instantly and without reservation that he is in error. Andrew Jackson
A subtle thought that is in error may yet give rise to fruitful inquiry that can establish truths of great value. Isaac Asimov
Insincerity is always weakness; sincerity even in error is strength. George Henry Lewes
No one who lives in error is free. Euripides
He who stops half way is only half in error. German Proverb