The C# language reference has an Annex A with an ANTLR grammar for C#. At the beginning of the annex it says This clause is informative. Does that mean that the grammar isn't necessarily correct?
I found one case where I am wondering about the correctness. As you can see in the screenshot a using directive can either occur near the beginning of a compilation unit or inside a namespace body. But I think this doesn't account for the fact that the body of a file-scoped namespace declaration doesn't have any curly braces and instead has a semicolon.