At one of my first jobs we had a big printer that would occasionally just stall in the middle of a job. Our in-house tech guy inspected it, but he could never troubleshoot it because it always worked when he was around. One time he walked in through the computer room door and it started up in the back of the room. Another time he was talking to our manager out front and we all heard it start up, all by itself. Finally he brought in a wallet-size photo of himself and taped it to the inside of the cover, facing the print mechanism. It never stalled again!