Simple & Direct:
* As wrong as night and day.
* As wrong as can be.
* As wrong as a broken promise.
More Figurative:
* Like a ship sailing off course.
* Like a puzzle piece that doesn't fit.
* Like a melody out of tune.
* Like a shadow cast in the wrong direction.
Specific to the Context:
* As wrong as stealing candy from a baby. (For something blatantly unethical)
* As wrong as a doctor misdiagnosing a patient. (For a serious mistake)
* As wrong as a misplaced comma. (For a small but impactful error)
The best simile will depend on the specific context and the feeling you want to convey.