The program

       program test
       integer :: fh = 71
       open(fh, file="f", form='unformatted', status='unknown')
!      close(fh)
       open(fh, file="f", form='unformatted', status='unknown')
       end program

fails with

At line 5 of file file-status-old.f90
Fortran runtime error: OPEN statement must have a STATUS of OLD

The F2003 standard says that if status='unknown', which is the same as if no status specifier is present, then the status is processor dependent. So technically this is not a standard conformance bug, but it's still a problem for some real codes, and it doesn't cost us anything to allow it.

Thanks to Bernhard Fischer for bringing up this problem.