See http://www.netbeans.org/issues/show_bug.cgi?id=68107:

Exception in thread "Debugger operator thread" java.lang.InternalError: Location with invalid code index
        at
com.sun.tools.jdi.ConcreteMethodImpl.codeIndexToLineInfo(ConcreteMethodImpl.java:167)
        at com.sun.tools.jdi.LocationImpl.getBaseLineInfo(LocationImpl.java:108)
        at com.sun.tools.jdi.LocationImpl.getLineInfo(LocationImpl.java:122)
        at com.sun.tools.jdi.LocationImpl.sourcePath(LocationImpl.java:187)
        at com.sun.tools.jdi.LocationImpl.sourcePath(LocationImpl.java:182)
        at
org.netbeans.modules.debugger.jpda.models.JPDAThreadImpl.getSourcePath(JPDAThreadImpl.java:193)
        at
org.netbeans.modules.debugger.jpda.ui.SourcePath.sourceAvailable(SourcePath.java:189)
        at
org.netbeans.modules.debugger.jpda.ui.SmartSteppingImpl.stopHere(SmartSteppingImpl.java:67)
        at
org.netbeans.modules.debugger.jpda.actions.CompoundSmartSteppingListener.stopHere(CompoundSmartSteppingListener.java:77)
        at
org.netbeans.modules.debugger.jpda.actions.StepIntoActionProvider.exec(StepIntoActionProvider.java:206)
        at org.netbeans.modules.debugger.jpda.util.Operator$1.run(Operator.java:142)
        at java.lang.Thread.run(Thread.java:595)
See also:
http://www.netbeans.org/issues/show_bug.cgi?id=54624