why did he do that ?
Service call goes out for a mainframe tape deck, engineer turns up later on & phones to be left in.
Someone walks down to main door and lets him and another man waiting next to him in.
A minute later second guy said "oh big red button" and presses it.
Operator and engineer look at each other, engineer said "why did he do that? "
Turns out it was some random nutter they let in.