[bityard]

A lot of services don't accept the '+' sign in an email address, even though it's perfectly valid according to RFCs. Part of this is ignorance of standards but I suspect part of it is deliberate. (To get the users' "real" addresses.)

[kiwijamo]

The worst examples are services that accept + when creating an account but does not accept + for logins. Believe it or not this has happened to me several times, and with large companies too. When companies started blocking + address specifically and/or being clever with removing the + part, I decided it was not worth using + addresses anymore.