In C++, any name that starts with and underscore followed by an uppercase letter and any name that contains two consecutive underscores is reserved for any use [lib.global.names] and any name that begins with an underscore is reserved in the global namespace. The intent for this, as explained by several people on comp.std.c++ is to allow C++ name mangling to result in valid names in C (because the two underscores restriction does not exist in C).
A naming convention I have been using for a few years now includes a rule about scope: anything with member scope has one underscore at the end; anything with global scope (including static class members and enumerators) have two. Technically, this breaks the requirement of [lib.global.names]. I have yet to find a standards-compliant compiler, however, that is not able to handle this correctly. If you know of one, let me know.