We see condensation and mold problems in new and older homes quite often. Invariably, we find out that the culprit is moisture, lack of insulation, or a combination of the two which results in an outside corner or edge "seeing" it's dewpoint.  This causes condensation to appear and eventually mold.  Sometimes, it can even result in drywall "pop", that is the drywall starts to sag or fall. 

Below is another picture.  This time of an outer wall, where the sidewall meets the ceiling.  In this picture, the mold is about 12 inches long.  The mold resulted from the attic insulation having been "blown away" from the top plate above the wall from outside air movement.  That is to say, the wind blew through the eve vents and pushed the insulation away from the outside edges in the attic.  This resulted in very little insulation being on the outside corners edge, and as soon as the weather got cold enough, and moisture high enough in the house, the moisture started to condense on the coldest surfaces first.  Normally you expect to see the condensation around and on windows, expecially on older homes, but with the insulation pushed back in the attic, this sort of thing happens all the time.  The method to stop this, is to install vent chutes (kind of like special air tunnels), in the eves to channel the air above the insulation.  Then you block off and insulate above the rest of the plate above the outside walls so that the wind does blow the insulation away.

This last picture, is of dimpling, or drywall "pop".  I know it is hard to see, but, with enough moisture condensing out in a house, it will start to saturate the ceiling drywall.  If it gets wet enough, the dry wall screws will start to pull through..