This drives me nuts. Forms that simply reject e-mail addresses with a + in them (perfectly valid according to the standards/RFC's - see RFC 2822 3.2.4 Atom - definition of atext).
Why would you want a + ? Well - in my case it allows me to have firstname.lastname@example.org - therefore allowing me to use unique addresses per site I use. All pretty normal stuff. But - web validators just don't seem to get it (and in some cases the companies behind it aren't interested in fixing it either). Only a short list to start with (there have been more instances of this than I can recall right now).
Sites that do the same thing (will be added to as I find more):
Reasonably uninteresting - but - really annoying.