I have had about four occasions now (2 within the last month) where I have found myself on someone elses mute list for no apparent reason. I have only found this out when the user buys an item from one of my vendors but never receives it. They then contact me about it, but when I try to contact them they don't recieve my IM's and the item when sent is declined. The only way around this is to contact them via one of their friends or via my alt account. In the most recent case the other user was able to find my name on the mute list and unmute it, which enabled me to contact and send them items normally with my main account.
At current I do not know how this bug can be reproduced as it seems to happen randomly.
Try putting up a sign warning people not to click mute.