Coding Horror describes it here: http://www.codinghorror.com/blog/2009/06/monty-hall-monty-fa...

http://mathworld.wolfram.com/PonsAsinorum.html

> An elementary theorem in geometry whose name means "asses' bridge," perhaps in reference to the fact that fools would be unable to pass this point in their geometric studies. The theorem states that the angles at the base of an isosceles triangle (defined as a triangle with two legs of equal length) are equal and appears as the fifth proposition in Book I of Euclid's Elements.

Generalizing the concept a bit, a pons asinorum is a critical concept in any field, such that if you don't grasp it you'll never comprehend the field as a whole. 0.999... = 1 is the critical point for the idea of the infinite series and, thus, an important part of how mathematicians use infinity.

And the concept of infinity seems to be the stumbling block for a lot of people, in the Internet arguments I've seen about this. A fair number of people seem to hold a religious fear of infinity, at least to the extent they apparently believe that it cannot be handled in a logical fashion; it is sacred and not to be mixed with such profane ideas as 3 or 12. Mix that with a half-formed notion of infinitesimals and some cod-Platonism and you have people seriously arguing that math is wrong and that they have inspired access to a true mathematical revelation.

Or they deny that the argument is mathematical at all and retreat into increasingly self-contradictory pseudo-philosophical arguments.

That's not a good sign. :)