I once had a page that prevented pasting, but last pass's password generator still worked. So I put in a long password using that, but when I clicked to register it came up with a blank error message. Turns out they had a 16 character limit that was only enforced when you typed in the box, so I had to count the number of letters they allowed me to type and then let lastpass generate a password of that length. Infuriating.