I could have tripped the breaker, either by deliberately shorting or figuring out which breaker it was (I've marked the breakers, but not very well--likely I wouldn't have known which to trip).  Probably should have, but I figured (not incorrectly, as it turned out) that with the dimmer pulled down it wouldn't be a huge deal to eat some juice through one hand.

Probably the simplest answer there would have been testing for contact by almost shorting the live wires and seeing if they arced, but I didn't think of it.  I should possibly own a multimeter.