Implicit conversions come to mind. They don't need to interact with anything to be confusing.

Consider this program:

    #include <iostream>
    using std::cout;
    using std::endl;

    struct C {
        int x;
        C(int _x) : x(_x) {}

        operator bool() const { return 0 != x; }
    };

    struct D {
        float f;
        D(float _f) : f(_f) {}

        operator bool() const { return 0.0f != f; }
    };

    int main() {
        C c(9);
        D d(2.0f);

        if (c == d) {
            cout << "Equal" << endl;
        }

        return 0;
    }

g++ 3.4.4 doesn't so much as warn that something is fishy here, even with -Wall.

Not only that, but c and d are considered to be equal.