> I know tons of C++ programmers who I'd happily trust to write code for me who would fall flat on their faces if asked to explain some detail of what std::forward does with an rvalue reference parameter bound to an lvalue argument.

That is true.

I can also remember other ones like differences between auto and decltype, or between auto and template parameter deduction across language revisions.

However, Java puzzles like the ones presented at Java ONE, or .NET changes in how native error handling changed on .NET 4.0, among many other examples, are also interesting as quiz questions.