“According to Special Relativity, an object in motion relative to you appears shortened. The faster it is, the shorter it appears. This is effect known as Lorentz-contraction. According to General Relativity, an object that has a sufficiently high mass-density in a small volume collapses to a black hole. Does this mean that if a particle moves fast enough relative to you it turns into a black hole? No, it doesn't. But it's a confusion I've come across frequently.
(Read on http://backreaction.blogspot.com/)