Back in August 2007, the creative team Frank Anselmo & Jayson Atienza at BBDO New York pulled a massive prank, where they gained access to the building before office hours, and "vandalized" over 500 shiny new iMac screens. Oh I would have loved to see peoples reactions. :))

Everyone in the company came into work to find their computer monitors vandalized with crayon scribbles.
To promote "Bring Your Kids To Work Day," childlike art was hand-drawn on acetate sheets, which were trimmed to fit the screens. Building security secretly unlocked over 500 offices the night before so the acetate sheets could be affixed to everyone's computer monitor.

Link to larger image here