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This happens when the voltages on the picture tube remains after sweep drive stop. Many early sets did nothing about it or happened to have circuits arranged in a way it didn't happen. Later sets included a circuit just to prevent this happening.
It kind of depends on when the TV was made.
Also it could be that capacitors or resistors have changed value in a way that allows this to happen.
You don't say what brand or what year this TV was made.
If the spot is not extremely bright it will not cause any harm.
You may wear the controls out before the picture tube.
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