This seems to occur only together with bug 1674039. I've seen between 1 and 3 occurrences detected in a single run. It looks like this often happens with toolkit/content/tests/browser/browser_media_wakelock_webaudio.js, but can also happen in other tests such as [after browser_delay_autoplay_webAudio.js](https://treeherder.mozilla.org/logviewer?job_id=325075954&repo=mozilla-beta&lineNumber=3056). Hypothetically this might happen if `AudioCallbackDriver::Init()` could be called again somehow while the `DataCallback()` is running, but I don't see a way that could happen. `FallbackWrapper::OneIteration()` is called only with a [null mixer](https://searchfox.org/mozilla-central/rev/8698fade12984b9a6a43a85a287a5f17e).
Bug 1683439 Comment 2 Edit History
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This seems to occur only together with bug 1674039. I've seen between 1 and 3 occurrences detected in a single run. It looks like this often happens with toolkit/content/tests/browser/browser_media_wakelock_webaudio.js, but can also happen in other tests such as [after browser_delay_autoplay_webAudio.js](https://treeherder.mozilla.org/logviewer?job_id=325075954&repo=mozilla-beta&lineNumber=3056). Hypothetically this might happen if `AudioCallbackDriver::Init()` could be called again somehow while the `DataCallback()` is running, but I don't see a way that could happen. `FallbackWrapper::OneIteration()` is called only with a [null mixer](https://searchfox.org/mozilla-central/rev/b2433a832c250c55255e0ee37d05192d04f20427/dom/media/GraphDriver.cpp#209).