The following code compiles with javac but fails in the verifier:

Object o = new MyValue();
synchronized (o) {
  throw new RuntimeException("synchronization on value type should not succeed");
}

Error: Unable to initialize main class Test2
Caused by: java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    Test2.main([Ljava/lang/String;)V @9: monitorenter
  Reason:
    Type 'QMyValue;' (current frame, stack[0]) is not assignable to reference type
  Current Frame:
    bci: @9
    flags: { }
    locals: { '[Ljava/lang/String;', 'QMyValue;', 'QMyValue;' }
    stack: { 'QMyValue;' }
  Bytecode:
    0000000: 102a b800 074c 2b59 4dc2 bb00 0c59 120e
    0000010: b700 10bf 4e2c c32d bf
  Exception Handler Table:
    bci [10, 23] => handler: 20
  Stackmap Table:
    full_frame(@20,{Object[#26],Object[#2],Object[#2]},{Object[#28]})

    Code:
       0: invokestatic #7 // Method MyValue."<init>":()QMyValue;
       3: astore_1
       4: aload_1
       5: dup
       6: astore_2
       7: monitorenter
       8: new #12 // class java/lang/RuntimeException
      11: dup
      12: ldc #14 // String synchronization on value type should not succeed
      14: invokespecial #16 // Method java/lang/RuntimeException."<init>":(Ljava/lang/String;)V
      17: athrow
      18: astore_3
      19: aload_2
      20: monitorexit
      21: aload_3
      22: athrow
    Exception table:
       from to target type
           8 21 18 any