It happened again last night, but as happens with these cases, the new member needed access to the site so rather than file a ticket and make them wait, we had them click the "Wrong?" link and processed it that way.
I'll try to create a test account and see if it happens on that one.
--
Quaiche - Addon Author, Druid, and Guild Leader for Benevolent Thuggery on Dragonblight
Alrighty. It seems that even though the fields are hidden, that the firefox beta is still filling in your current password into the "Password 1" field.
Click "Edit this User's Password" and just delete the entry, and then save. It'll work then. Until then, I'll incorporate a fix into the coming patch.