Can code that is valid in both C and C++ produce different behavior when compiled in each language?

Here is an example that takes advantage of the difference between function calls and object declarations in C and C++, as well as the fact that C90 allows the calling of undeclared functions:

#include <stdio.h>

struct f { int x; };

int main() {
    f();
}

int f() {
    return printf("hello");
}

In C++ this will print nothing because a temporary f is created and destroyed, but in C90 it will print hello because functions can be called without having been declared.

In case you were wondering about the name f being used twice, the C and C++ standards explicitly allows this, and to make an object you have to say struct f to disambiguate if you want the structure, or leave off struct if you want the function.

For C++ vs. C90, there's at least one way to get different behavior that's not implementation defined. C90 doesn't have single-line comments. With a little care, we can use that to create an expression with entirely different results in C90 and in C++.

int a = 10 //* comment */ 2 
        + 3;

In C++, everything from the // to the end of the line is a comment, so this works out as:

int a = 10 + 3;

Since C90 doesn't have single-line comments, only the /* comment */ is a comment. The first / and the 2 are both parts of the initialization, so it comes out to:

int a = 10 / 2 + 3;

So, a correct C++ compiler will give 13, but a strictly correct C90 compiler 8. Of course, I just picked arbitrary numbers here -- you can use other numbers as you see fit.

The following, valid in C and C++, is going to (most likely) result in different values in i in C and C++:

int i = sizeof('a');

See Size of character ('a') in C/C++ for an explanation of the difference.

Another one from this article:

#include <stdio.h>

int  sz = 80;

int main(void)
{
    struct sz { char c; };

    int val = sizeof(sz);      // sizeof(int) in C,
                               // sizeof(struct sz) in C++
    printf("%d\n", val);
    return 0;
}