The initial support for method reference parsing contained did not handle well an ambiguity between unbound method references and binary expression:

class Test {
   void test() { that(i < len, "oopmap"); }
   void that(int i, String s) { };
}

The above code failed to compile, as the compiler tried to parse the sequence 'i' '<' 'len' as an unbound method reference (whose qualifier is a generic type).