Once while building a rig, i was putting in a light unit, the spade connector had revealed the smallest patch of metal as it had come loose.

This touched the case, and everytime i powered up, it would short out. The first PSU i used died after that.

First of all i did not detect the fault, so i bought a new 550w PSU, this also cutout. Luckily this PSU has a failsafe and just cutout rather than died

Just check out that you have no bare wires, metal connectors showing or touchiing the case etc... There is a small chance of arcing from one possibly faulty component to another aswell.