[xsmasher]

To "not code in X correctly" is ambiguous, but I assume/hope the parent poster means that someone makes fundamental, non-syntax errors in their code - in C++ this would be something like returning a pointer to an object that's on the local stack.

If you're trying to filter for people can be productive in a particular language from anyone else, that's what you need to look for.

If you let the candidate pick their strongest language and they still make fundamental errors, you know they're not going to be immediately productive in any language.