When trying to recover a password using http://www.bibsonomy.org/reminder and entering an unknown email, the error message tells you to "Please enter a valid email address".
This error message is unclear. I was under the impression that there was an error in the form validation, making it think the address is syntactically incorrect. What it actually meant was that I gave the wrong email address.
(I've remembered my password in the meantime, so no bother there...)