Example:
newfile.h:
#include "newfile1.h"
struct S {
    int i;
    int j;
};

newfile1.h:
#ifdef MACRO1
struct S {
    int i;
};
#endif

newfile.cpp:
#include "newfile.h"

int main() {
    S s; // S links to S from newfile1.h
    s.j; // unresolved j
    return 0;
}

newfile1.cpp:
#define MACRO1
#include "newfile1.h"

newfile1.h is included two times from newfile.cpp (from newfile.h) and newfile1.cpp.
To newfile.cpp it's included without defines.
To newfile1.cpp it's included with MACRO1.
So #ifdef block in newfile1.cpp is active.
And newfile.cpp gets declaration of S from newfile1.cpp instead of newfile.cpp.

Situation is reproducible, but it depends on file parsing sequence.