If a procedure invokes a function then the function type must be declared in the procedure. However if both the procedure and the function are placed in a module, then the declaration of the function type must be removed as its meaning is changed.
C
FUNCTION funca(x)
REAL:: funca,
x, y REAL::funcb
...
y = funcb(x)
...
END function
funca
When funca and funcb are placed in the same module, funcb is automatically known to funca. The local declaration of funcb in funca then overrides the module declaration, and the program then looks for an external resolution for funcb. So normally this generates a linker error for an unresolved reference to funcb. If, however, the original funcb is still in a library, it resolves the reference and you unintentionally are using the old version of funcb.