Back in the CRT monitor days, I was working for a computer repair company. There was this particular client (in the defence industry) who had monitors that started flickering and having a greenish hue at its sides after a week.
Every week, we had to go to his office to swap the monitors and bring the faulty ones back to recalibrate (it was costly, but hey, its a Defence contract and those pay big bucks)
It didn't matter whether the monitor was brand new or recalibrate ones, it just started flickering and had greenish hue after a week, and it only happened in that room. Other monitors outside that room and in other levels were fine, thus the room was dubbed the Poltergeist Room (as they blamed spirits for messing with it).
One day after the monitor exchange, I returned to the office and my supervisor queried me as to why I didn't reply to his multiple pages (we were using pagers back then). I realised I was in the Poltergeist Room when the pages were sent and therefore did not receive any page. It then dawned on me, "Could it be some electro magnetic interference from another level directly above or below that was playing havoc".
I went back to the client the next day to tell him what I thought and he (being electronics trained) realised that above him was a defence lab carrying out EMF experiments, which could have caused the monitor problems. He got to work to build a simple Faraday cage to prevent EMF from getting to the monitor. Since then, the monitors worked perfectly.