Consider this code:
-----
import java.util.*;
class Test<T> {
    static void test() {
        Number s = new Test<Number>().first(Arrays.asList(0));
    }

    T first(List<T> t) {
        return t.get(0);
    }
}
-----

Running tl/langtools javac, tip 558fe98d1ac0 with -XDfindDiamond produces:
-----
$ javac -XDfindDiamond Test.java
Test.java:4: warning: redundant type arguments in new expression (use diamond operator instead).
        Number s = new Test<Number>().first(Arrays.asList(0));
                           ^
  explicit: Test<Number>
  inferred: Test<Object>
1 warning
-----

But removing the type argument leads to a compile-time error:
-----
$ javac -XDfindDiamond Test.java
Test.java:4: error: incompatible types: Object cannot be converted to Number
        Number s = new Test<>().first(Arrays.asList(0));
                                     ^
1 error
-----

Was originally reported as a NetBeans bug:
https://netbeans.org/bugzilla/show_bug.cgi?id=233129