I've always considered error handling to be the most important thing when it comes to knowing a language beyond the beginner level. Every language has it's own idiomatic ways from RAII in C++, finally/using in Java to the myriad of ways of handling return codes in C. It is also frequently undertaught in most programming language courses.
It is for this reason I despise seeing C/C++ on CVs. It implies that you don't have a strong foundation in either language as idiomatic code is so different between the two. By all means list them as two separate languages, but be willing to demonstrate sound knowledge of both, not the bastardised, resource leaking hybrid I see so often when the term C/C++ is invoked.