Invisibilty is a disease? :)
Yesterday I found myself a nice cloak of invisibility. Try it on. Cool! Come to town, go to shop and try to sell some stuff. Recieve message about shopkeeper do not deal with invisible man. Ok. Get out of shop, take cloak off - sell stuff. Fine. Next time - come to shop with cloak on, and try to remove it while in the shop. Cloak removed, but I am still invisible! I tryed to put it on and off several times - nothing works, I am invisible. Pray to gods - nothing.

You probably drank from a fountain or walk through a clound of magic vapour from a magic mushroom giving you invisibility. It wears off, that is unless you were also wearing a ring of invisibility. I think this kind of question belongs in the Tips Tricks and secrets or general discussion forum.