I was once on a client site on a call out when another issue got phoned into our office. A user (a particularly whingey one) was reporting one of their screens had failed. It made sense that I just pop to the machine in question rather than the support desk talking it through on the phone.
I sat at the desk and tried to ignore the whingey noises emanating from behind me. Looked at the monitor and saw there was no power light.
My first thought was actually "shall I pretend this takes me a minute to find or shall I just push the power button first?"
I split the difference and switched the monitor on after a little look around the desk.
From behind me, the immediate reaction was "well why was it turned off!?"