Legally, you could, IF you were negligent--which means "reasonably careless"--be held liable for the damage or additional damage done by you, not be pre-existing damage. Practically, if the other person wishes to try to sue you for the money, it be difficult to prove that you didn't do all the damage, if you leave a note admitting to knocked the mirror down. You have to make your decision in light of the fact that by admitting you did *some* damage, you may make it easy for that person to claim you did all of it.