I'm guessing he's referring to wildcards (http://cseweb.ucsd.edu/~atl017/papers/pldi11.pdf) and the fact that the type system allows this (which isn't really a complexity, but rather a deficiency):
Object[] foo = new String[1];
foo[0] = new Integer(4); // exception thrown here instead of a compile error.
Object[] foo = new String[1]; foo[0] = new Integer(4); // exception thrown here instead of a compile error.