How did this myth start?
There are a couple of theories. One is that one word has been removed from a previous version: "A watched pot never boils over." This phrase is very true, but there is some evidence against this phrase ever being in use (ie. Mary Barton (1848) by Elizabeth Gaskell). Another version (and the one which I feel is the most believeable) stems from when pots were so large that, to be checked, they needed to be taken off the fire periodically. This caused them to lose temperature and take a lot longer to boil.
Then again, there's always the theory that the phrase is supposed to be taken literally, in which case it is the actual pot that is supposed to boil, as opposed to the water within. In that case, the surrounding temperature would be so high that a human would be killed almost immediately, and it would be impossible to watch it, let alone watch it boil. (Conversely, watching it would also mean watching and regulating the temperature, allowing the pot to stay unboiled.)
Anyhow. I, personally, say WATCH THE FUCKING POT!!!1! Too late. Dinner's served ruined.