You probably know about how the mailboxes on Nuketown have names of characters on them, but I found another easter egg.

First go into Nuketown and choose spectate in team select, then go to the mailbox that says Hoccat in the part of Nuketown that you can't get to. It's to the right of the barbwire fence.

Then bareley move forward until you're in the mailbox and it will say Rex.