ISTR someone complaining about a program where someone "fixed" a problem by changing (say) R1 EQ 1 to R1 EQ 4 This might "work" but is of course asking for big trouble down the road! As Shmuel suggests, looking at the assembler output would be helpful in pathological cases like this.
GEICO uses 2FA whose values are alphanumeric and include 1, I, L, 0, O. Brilliant. While working helpdesk, someone I know was brought a FORTRAN program with a problem. While looking at it, something kept bothering him; finally he figured it out: the first word of EVERY comment started with "C", so the comments just looked like text. Weird!
