Switching left and right mouse buttons was a favourite where I used to work. Then one day I noticed the the keys on the keyboard felt 'odd' so I looked at it closely and realised someone had moved the 't', 'n' and 'u' keys to the bottom row next to the 'c' key to spell a very naughty word. It turned out they'd done it several weeks before and I just hadn't noticed as I'm pretty much a touch typist nowadays!