I'd have to vote "Never" but thats only because I had to invest in a keyboard tray (you know it screws to the underside of the desk and slides and and out when you need it)

Allen was ALWAYS dancing on my keyboard tray and more than once pushed one of the F keys that seriously screws up whatever it was that I was doing. Late at night you might hear atrange, insistent beeping moises... you turn on the light to find Allen drapped across the keyboard and hitting several keys!

So, No, but Allen doesn't get the opportunity anymore!