Usually cold solder joints. When the manufacturers started switching to ROHS they used solder with no lead content and this requires a slightly different soldering technique. When it's not soldered properly then the solder breaks and you end up with an intermittent connection (what most people would call a short, though that is totally incorrect and a pet peeve of mine). A lot of times you can take something apart and everything will be fine other times you damage physical connections and they will crack and need to be reflowed with solder. That is the most common cause. Your description of forgetting to add an insulator could be the cause or might be a coincidence.
Jeff
