the other issue is when someone attempts to use non-leaded solder and add it to a leaded solder connection. all solder joints should be completely removed and then new solder used. non-leaded and leaded solder do not mix well. additionally, old solder and new solder mixed (leaded or otherwise) can cause issues as well.
that and freeplay3 comment about using a cheap soldering iron that didn't get hot enough to properly flow the solder
a good solder connection should have a shiny sheen to it. dull solder connections are highly suspect of being a cold solder connection and/or bad solder