(In this case, being unable to submit the form at all is actually the best outcome, because it clearly tells the user that something is wrong while stopping them from doing any damage.)
For example, I once tried to create an account on such a website. The account creation form asked for your login name and your password; I picked a login and left the password field blank, assuming that either the system would spit an error at me or it would create a random password. Instead, it created the account with a blank password, which it turned out wasn't supposed to be possible, and various bits of the site were not too usable as a result. Including the password change form.
Unsurprisingly, I haven't really been back since.