If you think the mirror's lying and telling you there's some part of yourself there that really isn't, just perform this little experiment:

Hold what appears to be the part in question with your left hand. Using your right hand, hack at the questionable part with a K-bar knife. If the part comes off and you begin to bleed profusely, the mirror was telling the truth, more or less. It still may have been lying about the exact appearance of the part, but that will be of little consequence since you'll soon be losing consciousness due to blood loss.