Registration web pages seem to have this new thing - "Please confirm email address". Ah, I think you mean shift-tab to go back to the previous field, command-c (or control-c as appropriate) to copy the email address I already put in, tab to go to the next field, and command-v to paste. And don't even think about trying to disallow copy and paste.
Mind you, I regularly get emails which are meant for other people. I had one the other day from Dell for a David Stuart (or is it Stuart David?) about returning a defective part: I get some from a pseudonym where I know the real address so I forward it to the real Sue Donym: and I received this cracker the other day:
Eric and I looked and determined the porn URL's began w/post #145 dated 12/29/07.