match_field validator is busted in attached fields

[hurrymaplelad]

To reproduce:

1. Open up an example with password confirmation.
2. Type 'aaa' in password.
3. Type 'bbb' in confirm.
4. Error message appears.
5. Change password to 'bbb'.
6. confirm is still invalid.

Needs to know when the matched field changes. Should this be a form level validation?