Y2K was one of the few foreseeable major situations in human history actually predicted and prevented. Nonetheless:

• There were loony end-of-the-world predictions. Many of these came from fringe religious groups. They predicted things like planes crashing, cars not starting, and the like. There was a run on generators for example.
• There were some pretty nasty things that showed up in testing, including surprises with sewage systems and power plants. The comp.risks archive would be a good place to start.
• Some major real stuff happened - like a military satellite data outage
• and lots of little things