ADDITIONAL SYSTEM INFORMATION :
Picked up _JAVA_OPTIONS: -Dawt.useSystemAAFontSettings=gasp
java version "1.8.0_151"
Java(TM) SE Runtime Environment (build 1.8.0_151-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.151-b12, mixed mode)

A DESCRIPTION OF THE PROBLEM :
The following sample program presents different results when executed multiple times.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Compile above program
2. Run it using Oracle HotSpot java

ACTUAL -
Sometimes null, sometimes ClassCastException.

---------- BEGIN SOURCE ----------
public class A {
   public static void main(String args[]) {
      ConcurrentSkipListMap concurrentSkipListMap = new ConcurrentSkipListMap();
      System.out.println(concurrentSkipListMap.put(new Object(), new Object()));
   }
}
---------- END SOURCE ----------

FREQUENCY : often