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.
This has happened to me on many occasions, even with bigger sites which "should know better". Tis why I'm always slightly paranoid about entering in long passwords when signing up to new sites.
