Show me a language with >1 implementation where at least two are completely compatible. Common Lisp has one of the best track records in this aspect. C, C++ and JavaScript are all far worse.
C++ is similar with regards to C code. They're compatible enough that a lot of people will write "C/C++", but they're still incompatible enough that you'll get yelled at by a lot of people if you write "C/C++".
