Steps to reproduce:

1) Compile the following short program with --enable-checking:

	public class CLASS_ONE {
		public string field_one;
		public CLASS_ONE() { field_one = "test"; }
	}
		
	/***********************************/

	public void do_something_with_class_one(ref CLASS_ONE victim) {
		victim.field_one = "test failed";
	}

	/***********************************/

	void main()
	{
		CLASS_ONE c_1 = new CLASS_ONE();
		do_something_with_class_one(ref c_1);
	}

...and attempt to run it.

Notice that an error after the following form appears:
   ** (process:2674): CRITICAL **: do_something_with_class_one: assertion `IS_CLASS_ONE (victim)' failed

Compiling down to C code (with the command valac -C --enable-checking name_of_file.vala) reveals the following code in do_something_with_class_one():

	void do_something_with_class_one (CLASS_ONE** victim) {
	   CLASS_ONE* _tmp0_;
	   gchar* _tmp1_;
	   g_return_if_fail (IS_CLASS_ONE (victim)); // <---- problem
	   _tmp0_ = *victim;
	   ...

IS_CLASS_ONE will fail, because it's expecting a pointer to a CLASS_ONE, rather than a pointer to a pointer to a CLASS_ONE.