I added an overhead light to a room yesterday, there was no light in the room before, only a switched outlet. I used an "old work" ceiling box and ran a wire from it, to the switch box on the wall. (Minimal drywall work, since the floor above is being remolded - I found the top plate up there and was able to hit the box from above)
Once the switch was installed, I turned the breaker on and went back upstairs to flip the light on. When I flipped the switch, there was a pop sound from the switch box, and the breaker tripped.
I knew that the problem was with the new wire that I ran, because I didn't come into play, until I flipped the switch.
The problem was right in the metal switch box - the built-in cable clamps can have sharp edges, and one of them nicked the insulation on the hot wire going to the light fixture.
1. watch those sharp edges and protect wire insulation jackets
2. make sure your metal boxes are grounded
3. make sure your wiring boxes are sealed to contain the large arc that occurs with a short to ground
This was a simple job and a simple fix, but that's what made it an eye opener, don't get complacent.