I did this too when wiring a fluorescent light. Someone had monkeyed with it and undid a bunch of wires, and when reattaching them I didn't know which of the wires coming from the switch and the ballast went to the line power. I took a guess. As soon as I flipped it on, boom, power went out and the breaker in the basement tripped. I forget what I did (this was 7 or 8 years ago when I was still learning) but I think all I did was make a looped circuit from the switch to the ballast and back to wall power and it was basically a dead short.