Granted, we've been presupposing C, but:

In C++ you have a simple out by overloading T::operator&, allowing you to alter the result of the RHS of that equation and break it.

There's also a little bit in the standard prohibiting comparing pointers to different arrays - a leftover from the 16-bit era of near and far pointers I suspect - but that appears to only apply to relational comparisons in the C++11 standard:

http://stackoverflow.com/questions/9086372/how-to-compare-po...

I either misremember this applying to == and != as well (likely), or this changed since the C++03 standard that I'm used to referencing (unlikely). I'm also unsure if C has an equivalent rule. A fun note from that SO link:

    std::less<T*>

guarantees more than

    operator<( T*, T* )

enabling

    std::map<T*,...>

et all.

(edit: formatting)