It should go without saying that all of your mains cables and plugs should be in good physical condition. Earths should be connected where required and the correct rating of fuse fitted.
These matters should be formally checked once a year, and of course you would keep an eye out for problems every day.
But what about inside the plug? Unless the plug is moulded on there is the possibility that a wire could have worked loose inside - possibly the earth wire.
It isn't all that uncommon to find that the ends of the wires inside the plug have been tinned with solder. Obviously the person who fitted the plug thought it would be neater that way.
Unfortunately, solder has the habit of behaving like a liquid, except it flows very, very slowly. A tinned connection will work loose eventually.
If you have any doubt about your ability to check and service mains connections, you should have your mains wiring checked by a qualified electrician.
While he or she is in your studio, have the earthing of the mains sockets tested too.
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