Poor old Matt has been struggling to remember to turn off his desk fan at the end of each day, so Mary decided to take it (the fan) into her own hands, and mete out her own particularly sneaky form of justice.
She may try to tell you that this was an environmentally responsible act, coming down hard on an outrageous waste of electricity, but I think she just fancied a mischievous act of revenge.
UPDATE : The screws and fuse have now been returned to Matt