A couple of roles back, I once made the mistake of forgetting to lock my desktop before going to lunch... okay, I may have sent a couple of prank emails from similarly unlocked machines prior to this, and possibly have changed a couple of desktop wallpapers... you know, nothing malicious.
Oh, kharma... how sharp your teeth can be.
I got back and found that pretty much everything had been hit. Not just the things I had half expected on the way back from the sandwich shop when - in a moment of dawning horror - I realised what I had done, but everywhere.
I got the email signature switch before it could do any damage - but there are a few commits in that company's git repos that will forever immortalise that day before I spotted the vandalism...