In some cases an internal procedure will use a module procedure instead of another internal procedure (in the same host unit) with the same name. In the example below, R(x) calls f(x), but it uses the module's f(x) instead of the one internal to g(x) (as probably intended).

module test_functions

contains

subroutine f(x)
  implicit none
  real, intent(inout) :: x
  x = 2 * x
end subroutine f

subroutine g(x)
  implicit none
  real, intent(inout) :: x
  call R(x)
  contains
    subroutine R(x)
      implicit none
      real, intent(inout) :: x
      call f(x)
    end subroutine R
    subroutine f(x)
      implicit none
      real, intent(inout) :: x
      x = 3 * x
    end subroutine f
  end subroutine g
end module

program scope
  use test_functions
  real :: x = 3.0, y = 3.0
  call f(x)
  call g(y)
  write(*, *) x, y
end program


Compiling and running with gfortran 5.4, 7.4, 8.1, gives:

   6.00000000       6.00000000

Compiling and running with ifort 18.0 gives:

   6.000000       9.000000    


This does not happen if f(x) is a function instead of a subroutine, or if the call to f(x) is made directly in g(x) rather than in R(x).