This reminds me a little bit of a funny story from my early days at LBL in Berkeley.
I worked for several years in Building 70A and one of the labs there installed a new Zeiss SEM which they got up and running fairly quickly. Once the Zeiss engineers left they started using the SEM, but every once in a while the instrument would simply not produce an image for hours at a time. And then the imaging would suddenly be OK again. Went on for weeks like this. So they had the engineers come back in and one in a while the engineers would see the issue also, but were not able to figure out the problem.
I should mention that this building was constructed on the side of a very steep hill and had many entrances/exits. So one could enter or leave the building from several different "floors". One day my boss, who at the time who would sometimes explore the building by leaving from different exits, was wandering around and came across a lab he had never seen before, so as he was peeking in at the open door, one of the scientists asked if he could help, and my boss said he was just curious, so the guy invited him in for a tour. As they went around the room, the scientist explained each of the pieces of equipment and eventually they came to this big doughnut shaped thing in the middle of the room, and then the scientists said: and this is our new super conducting magnet!
At that point my boss realized that the SEM which they were having intermittent problems with, was located in the lab above, about 10 feet directly over the superconducting magnet! Of course every time they turned that darn magnet on the beam went away!