Found two more instances of partially broken email address handling: Mailman (version 2.1.9) does not allow user+suffix@some.domain addresses in `remind me' link, and Ebay allows to add such an address, but confirmation screen does not allow it.
The real annoyance here is that it works, but up to the point. For now, I maintain about 80 (eighty!) aliases for various sites which do not allow plus signs in email addresses.
For those of you who do not know what this is for: Cyrus IMAP server uses it for automatic filing (Fastmail.FM is one commercial-grade email hosting using it). For purists: RFC822 and successors allow almost any text on left side of @-sign (even spaces if quoted.) And yes, Gmail handles it just fine.