A soldered joint that wasn't made right:
the solder was melted on to the metal, but the metal was
not hot enough to melt the solder, meaning that the solder didn't really flow and is not bonded to the metal.
A cold joint is a mechanical joint at best:
it may hold together for a while, but it will eventually break loose, and it will never hold water or conduct electricity (whichever the case may be) very well.

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