Perhaps one of the world's most bug-free programs. Its version number is asymptotically approaching pi (currently at about 3.14159). DonaldKnuth? offers rewards to people who find bugs in TeX: checks for sums that are increasing powers of two (next bug to be found will receive $327.68).
Knuth reports on [his web site]: "My last will and testament for TeX and METAFONT is that their version numbers ultimately become $\pi$ and $e$, respectively. At that point they will be completely error-free by definition."