[Nekobai]

Indeed! And even worse, some services will happily accept "+" in email fields, but then some part of the service fails to encode the "+" sign correctly, so some features may be broken in unexpected ways.

Sometimes you can't even contact Customer Services because "your account doesn't exist" (because you cannot feed the correct email address to their customer service site).

Thankfully it's rare, but when it happens it's extremely infuriating.