Floating-point arithmetic

If I can't be a number, none of you can!

(See also cascading nulls)

for the math homies, you could say that NaN is an absorbing element

In Kerbal Space Program your ships sometimes catch the NaN virus. If one fuel tank level is reading NaN then whatever you do DON'T try and fill it from another (full) tank. I'm not sure if it can spread to physics (thrust, mass, etc) EDIT: Yes it can happen to physics, oh dear.

I wonder what would happen if you landed a NaN-infected spaceship on a planet.

It's infectious!