Breaker Trips When Light Switch Is Turned On

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.

Ideas?