LSpark
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Certain type of lights will cause this, such as fluorescent lights, as they are an inductive load. You will normally only notice this if it's very dark.
Best course of action is just to replace it with a quality branded switch as The_Trician suggests, this may solve the problem for you, however it's possible it may not, in which case providing there are no loose wires this is nothing to worry about.
Light switches have only a micro-gap inside them, so when you turn them on it draws an arc, this is what you see as the flash.
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