It just says "wrong password" and you'll be guessing at which random character did it cut the password. Luckily sometimes it's just a stupid html verification form that can be disabled in the console and be submitted anyway.

My e-mail provider does this. I wanted to change my password to some 64 character long generated string. It accepted, but I could not log in after that. After a few tries, I found the reason and, after another few tries, also the limit at which it gets truncated: 16 characters! God, how I hate them for this...