Metcalfe's Law that a network system's value is proportional to the square of its user population
Predicting in 1997 that the Internet would soon fail due to traffic or he would eat his words -- and did, mushing his paper with liquid in a blender a year later
Predicting that Microsoft Windows 2000 would kill Linux, likening the open source movement to "utopian balderdash" with "open sores"