99% of the people on Earth have a higher than average number of legs.

No flaw with math/stats here, that can actually happen. I think you're confusing average/mean with median (50-percentile).

PS: I'm not sure where he's getting his numbers though

Depends which average it refers to any of the means, or the media, or the mode can be “average” (most typically, though, it means the arithmetic mean), and it's only impossible in terms of the median.

You've never been on a project that was 80% done and 80% to go?

Not native speaker here, but I think, he just meant that, most programmers and drivers thinks they are better in respective things then, they really are.

It is just like letting programmer build a house. It is just a really bad idea.