I went to help install a chandelier today and this is the sequence of events:
There was no light fixture in pace when I arrive. The previous owners had removed a fixture and my friend, who's lived there for two years, just got around to buying one.
At the fixture box, there was a black, white, and ground, all with wire nuts on them.
I connected the new fixture. Before I hung it, I put a bulb in and tested it. Everything worked fine.
I affixed the new light to the box and tested again. I heard a loud pop at the switch (a dimmer switch). The breaker at the panel had tripped.
Thinking it was the dimmer, I purchased a new dimmer and tried again. Another pop and the breaker tripped again.'
-After resetting the breaker, I bypassed the switch completely and the light worked.
I then purchased a single pole switch (no dimmer). I installed it and tested the light. It worked.
I put the switch back into the wall and tested again. It tripped the breaker.
I took the switch back out of the box and tried again. Tripped the breaker again.
From everything that's visible, there are no shorts.