Signal exceptions if start/stop is called out-of-order or too many times.

Source/modules/webaudio/AudioBufferSourceNode.idl

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 25 matching lines @@
     readonly attribute unsigned short playbackState;
 
     readonly attribute AudioParam gain;
     readonly attribute AudioParam playbackRate;
 
     attribute boolean loop; // This is the proper attribute name from the specification.
 
     attribute double loopStart;
     attribute double loopEnd;
 
-    [MeasureAs=WebAudioStart] void start(optional double when, optional double grainOffset, optional double grainDuration);
-    void stop(optional double when);
+    [RaisesException, MeasureAs=WebAudioStart] void start(optional double when, optional double grainOffset, optional double grainDuration);
+    [RaisesException] void stop(optional double when);
 
     [DeprecateAs=WebAudioLooping, ImplementedAs=loop] attribute boolean looping; // This is an alias for the .loop attribute for backwards compatibility.
 
-    [MeasureAs=LegacyWebAudio] void noteOn(double when);
-    [MeasureAs=LegacyWebAudio] void noteGrainOn(double when, double grainOffset, double grainDuration);
-    void noteOff(double when);
+    [RaisesException, ImplementedAs=start, MeasureAs=LegacyWebAudio] void noteOn(double when);
+    [RaisesException, MeasureAs=LegacyWebAudio] void noteGrainOn(double when, double grainOffset, double grainDuration);
+    [RaisesException, ImplementedAs=stop] void noteOff(double when);
 
     attribute EventHandler onended;
 };