Ha - The first link on Google from RWebb's link - safety - third item down - Don't assume black is "LINE" and white is "LOAD"
How to Work with Electricity Safely - For Dummies
Saturday changed out a 220 V pressure switch on a house water pump and kept blowing the breaker until I figured out that one of the Romex 110 lines had two hots. 110 on a white and 110 on the black. The other Romex carried the two neutral lines (one black one white) back to the box.
I double checked there was no power at the device initially but burnt a whole lot of time scratching my head and re-reading directions because I made the assumption that the wiring followed convention.