CiviCRM will not recognize valid email address
The client-side validation in CiviCRM will flag any invisible or space characters which get pasted in along with an email address or URL. The non-printing characters are the trickiest to find, including things like carriage returns and line feeds that seem to get swept up in some copy-paste operations. I encounter this problem relatively frequently, but it is hard to pinpoint the exact combination of platform, browser, and source.

Therefore, if I have an e-mail address or URL which doesn't seem to take even though it appears to be well-formed, I "clear" the characters by pasting someplace else first, like the To: field in an e-mail or the URL bar in the browser. This strips out the non-ASCII characters, and I can then re-select the desired text and paste it into CiviCRM as usual.