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Typically dry solder joints where the surfaces of both connections are not tinned properly. Real bad ones form like a tube around the legs of components and over time oxidation within that forms a barrier. A good belt can re-form the contact albeit temporarily. Likewise similar problems happen with connectors and leads. Also crimped wire connections can oxidise and a jolt can re-connect by piercing the oxidation.
Loose non housed connectors , same as before. Switches and anything mechanical can also exhibit the same problems.
Sometimes also called cold solder joints, i've fixed a few with re-soldering after inspection, but it's a bit of an art to spot them and you need to be good to solder too. Luckily I worked on a missile guidance production line with 50 women but even then a good whack was the best solution (only for the argentinian export market)
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