I was listening to Scott Hanselman's MSR at PDC podcast and he discussed a tool called Chess which is designed to work with Team MSTest in VS2008 to track down Heisenbugs. I'd never heard of this type of bug being called a Heisenbug before but have experienced it plenty. Simply stated, a Heisenbug is a bug that dissappears when the code is run through the debugger.
The name Heisenbug appropriately comes from the Heisenberg uncertainty principle which states that the position and momentum of a particle cannot both be known simultaneously.
I loved Hanselman's Heisenberg joke:
Heisenberg get's pulled over by the police.
Officer: "Do you know how fast you were going?"
Heisenberg: "No, but I know exactly where I am."