It happened to colour TVs, too....
Back in the late 60s, between leaving college and starting a "proper" job, I worked as a salesman / delivery driver for a local shop that supplied TVs, washing machines, Hi-Fis, etc. One of our customers had just bought one of these new-fangled colour TVs - a dual standard 405/625 model, if memory serves correctly. These early sets were very sensitive to stray magnetic fields - even the Earth's - and had to be carefully aligned by a service engineer in situ.
Our resident alignment expert went out with it on delivery, carefully de-gaussed the screen and carried out all the usual purity and convergence adjustments, leaving the customer with a crystal clear picture.
A week later, the customer called to complain his TV had gone screwy. The service engineer went out again, and sure enough, the purity and convergence had all gone to pot. He carefully re-aligned it all, and left the customer with a perfect picture again.
Exactly a week later, the same problem ensued! By now the customer was getting a little irate (these sets were very expensive!), and the service engineer very puzzled!
Since the problem always seemed to happen on a Thursday, the service engineer convinced the boss to let him go and sit in the room on Thursday - all day if necessary - to see what was going on.
He arrived at 9 o'clock sharp, and the TV was fine. It stayed that way until around 1130, when the cleaning lady arrived and proceeded to hoover the room containing the TV, pushing the hoover with its heavy and powerful electric motor under the TV! Needless to say, the picture immediately went bananas, and required a careful de-gauss and re-alignment to restore proper operation!
The cleaning lady was very apologetic, but in truth it wasn't her fault! No-one had anticipated the effect a powerful vacuum cleaner might have on a CRT!
Later TVs had much better screening and better built in de-gaussing systems, and of course, modern displays aren't affected by stray magnetic fields. But back then, it was all one big learning curve......!
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Pete