http://abcnews.go.com/Technology/space-station-resupply-mission-delayed-mold-found-bags/story?id=36895722
Simply put, they noticed that there was mold on the bags used to contain supplies meant for the upcoming ISS resupply. They are going to thoroughly examine the contents and such to ensure no contamination, which is pushing the launch back. They say that the mold wouldn't have grown terribly well in the dry cool air of the station, so there is no real worry about problems this or other missed situations might have caused.
Not sure why, but this reminds me of when I read that apparently all the ISS members have their little "secret stash" of goodies hidden away somewhere, and any found stash that they think might be from a previous team is fair game and a bit like christmas. Never know what you might find in space!