[ojbyrne]

I often use "There are 2 hard problems in computer science: cache invalidation, naming things, and off-by-1 errors."

[thedufer]

I prefer "There are 3 hard problems in computer science: cache invalidation and naming things."

[r00fus]

My favorite on this vein is: "There are 10 types of people in the world: those who understand binary and those who don't."

[elwell]

I like this variation:

  There are 10 type of people in the world:
  Those who understand binary
  Those who don't
  And those who count from zero

[athaeryn]

Or:

There are 10 types of people in the world: those who understand binary, those who don't, and those who didn't expect this joke to be in base three.

[peapicker]

I prefer, "There are 10 types of people in the world: those who understand hexadecimal and F the rest."