Oh yeah, and I can kill Spies when they go invisible.
Here's a funny little story about me (Spy) and an Engineer.
It was at 2Fort, and I was diguised as a Demoman. Then I walk up to a sentry, about to sap, but then the Engineer who made it went over to me and hit me twice, then I try to escape through the sewers. I hide behind a corner with him following me, I think to myself "He's come to finish the job!", then I go invisible, he walks srtaight ahead, tries to find me and then I backstab him!
It was fun. :P