From http://stackoverflow.com/questions/32585425/java-nashorn-inconsistent-binding-behavior-is-this-a-bug:

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import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import jdk.nashorn.api.scripting.NashornScriptEngineFactory;

public class JsBugTest {

    public static Object resolve(final ScriptEngine engine, final String script) {
      Object r = null;
      try {
        r = engine.eval(script);
      } catch (final Exception ex) {
        System.out.println("exception: " + ex.getMessage());
        r = null;
      }
      return r;
    }

    public static void runTest()
    {
       final ScriptEngine jsEngine = new NashornScriptEngineFactory().getScriptEngine("--log=fields", "-ot=false");
       final String script = "DataA + 'foo';";
       final Bindings binds = jsEngine.getBindings(ScriptContext.ENGINE_SCOPE);
       Object ret;

       for (int i = 0; i < 12; i++) {
         binds.remove("DataA");
         ret = resolve(jsEngine, script);
         if (ret != null) {
           System.out.println("Iteration " + i + ": Returned value should be null but is: \"" + ret + "\"");
         }

         binds.put("DataA", "foo");
         ret = resolve(jsEngine, script);
         if (ret == null) {
           System.out.println("Iteration " + i + " failed");
         }
       }
    }

    public static void main(final String[] args) {
      JsBugTest.runTest();
    }
}
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The program prints:

exception: ReferenceError: "DataA" is not defined in <eval> at line number 1

but then on iteration 8 starts printing:

Iteration 8: Returned value should be null but is: "undefinedfoo"

Running with "--log=fields" we see:

exception: ReferenceError: "DataA" is not defined in <eval> at line number 1
[fields] Megamorphic getter: dyn:getProp|getElem|getMethod:DataA(Object)Object@jdk.nashorn.internal.scripts.Script$\^eval\_ DataA false
Iteration 8: Returned value should be null but is: "undefinedfoo"

That is, we switch the call site to a megamorphic getter right before the incorrect behaviour starts happening.