http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2017-May/023453.html

super class Foo
version 52:0
{

...
static Method a:"(Ljava/lang/Object;)V"
stack 2 locals 3
{
aload_0;
monitorenter;
                aconst_null;
         monitorenter;
                aconst_null;
         monitorexit;
aload_0;
monitorexit;
return;
}

When calling Foo.a, https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-2.html#jvms-2.11.10 says that NullPointerException should be thrown. However, the JDK throws IllegalMonitorStateException:

===============
OUTPUT from JDK8u92:

java version "1.8.0_92"
Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)

Calling Foo.a()
java.lang.IllegalMonitorStateException
at Foo.a(Foo.s)
at A.main(A.java:16)
Calling Foo.b()
java.lang.NullPointerException
at Foo.b(Foo.s)
at A.main(A.java:22)