I think it's interesting how language changes over time. What something used to mean, it no longer means. Take these for example...
Tea Party: If your little girl says she's going to have a tea party, do you now assume that she's starting a racist political movement?
Taking the piss: Something to do with R.Kelly?
Mission Accomplished: You pretty much sound like a dumbass saying this now, thanks to dubya.
Go to pot: Go to California?
Shell Shocked: Someone after a hunting trip with Dick Cheney.
Stiff Upper Lip: This is probably a procedure you can have done now.