This was from code originally from about 1978ish, which compiled on a Microsoft Fortran compiler from the 2000s. It looked something like (without indents):

    DO I=1,10000
      ...
      IF (QQQ .LT. RIX70) THEN
         CALL IXNARF( I, J, R1, R7, XL23 )
         GOTO 23
         ...
      END IF
      111 CONTINUE  
    ...
    END

IXNARF could change 'I' (because Fortran 77 passed arguments by reference not value) and sometime after line (er... "card") 23 it might jump back into the loop at 111.

oh and there was like three comments in the whole several thousand lines, including my favorite at the top of one of the main loops:

    C  CHUGGA CHUGGA CHUGGA

> Change the loop counter? That's supposed to be impossible in Fortran, isn't it?

It is impossible inside the loop, even in F77.

But if a global variable is used as a loop counter, and the variable is altered in a subroutine, then the code altering the variable is not inside the loop. At least not in the eyes of the Fortran compiler.

    $ cat test.f08

    program test
        integer i
        do i = 1,3
            call sub
            print *,i
        end do
    contains
        subroutine sub
            i = 3
        end subroutine sub
    end program test

    $ gfortran --std=f2008 test.f08 
    $ ./a.out 
           3