The C# compiler accepts the following program as syntactically valid:
static void Main(string args)
Console.WriteLine("Hello " + (args: "World!"));
However, according to my understanding of the C# grammar, it is not a valid program.
The C# compiler translates this into a CLR assembly which throws an InvalidProgramException at runtime (“Common Language Runtime detected an invalid program.”).
(By the way, the use of the word “args” is irrelevant. The C# compiler accepts this program with any identifier before the colon.)