Back in 1992 I had to fix a problem in a PL/1 program that was checking if it was a leap year. The program was written in 1986. The code (I forget the syntax, too long ago) was something like :
leapyear = false
if (year = 1988) then leapyear = true;
So it ran for 6 years until it failed. I guess the author never expected it to run that long.
AC, because I still work for the same company, although PL/1 is long gone :)