I'm not certain where this phrase came from. All I know is that it's really really true.

Once a false assumption has been entered into your argument you are fucked. No matter what you do, you will be wrong. Often wrong in interesting and spectactular ways.
The moral? Make a habit of challenging all of your base assumptions. It could save you from something nasty, and will often provide remarkable insight into the nature of the universe, or at least human nature.