C# gotcha

Consider you have the following code:

SomeClass reference = null;  
Console.WriteLine(reference.Echo("echo"));

Your first thought might be NullReferenceException. Ha, think again! What if you have an class defining an extension method like this:

public static class SomeClassExt
{
    public static string Echo(this SomeClass x, string str)
    {
        return str;
    }
}

You have been warned :)