--- a/dom/media/tests/mochitest/test_getUserMedia_constraints.html
+++ b/dom/media/tests/mochitest/test_getUserMedia_constraints.html
@@ -42,16 +42,22 @@ var tests = [
     constraint: "mediaSource" },
   { message: "unknown mediaSource in audio fails",
     constraints: { audio: { mediaSource: 'uncle' } },
     error: "OverconstrainedError",
     constraint: "mediaSource" },
   { message: "emtpy constraint fails",
     constraints: { },
     error: "NotSupportedError" },
+  { message: "Triggering mock failure in default video device fails",
+    constraints: { video: { deviceId: 'bad device' }, fake: true },
+    error: "NotReadableError" },
+  { message: "Triggering mock failure in default audio device fails",
+    constraints: { audio: { deviceId: 'bad device' }, fake: true },
+    error: "NotReadableError" },
   { message: "Success-path: optional video facingMode + audio ignoring facingMode",
     constraints: { audio: { mediaSource: 'microphone',
                             facingMode: 'left',
                             foo: 0,
                             advanced: [{ facingMode: 'environment' },
                                        { facingMode: 'user' },
                                        { bar: 0 }] },
                    video: { mediaSource: 'camera',

--- a/dom/media/webrtc/MediaEngineDefault.cpp
+++ b/dom/media/webrtc/MediaEngineDefault.cpp
@@ -90,16 +90,22 @@ MediaEngineDefaultVideoSource::Allocate(
                                         const MediaEnginePrefs &aPrefs,
                                         const nsString& aDeviceId,
                                         const nsACString& aOrigin)
 {
   if (mState != kReleased) {
     return NS_ERROR_FAILURE;
   }
 
+  // Mock failure for automated tests.
+  if (aConstraints.mDeviceId.IsString() &&
+      aConstraints.mDeviceId.GetAsString().EqualsASCII("bad device")) {
+    return NS_ERROR_FAILURE;
+  }
+
   mOpts = aPrefs;
   mOpts.mWidth = mOpts.mWidth ? mOpts.mWidth : MediaEngine::DEFAULT_43_VIDEO_WIDTH;
   mOpts.mHeight = mOpts.mHeight ? mOpts.mHeight : MediaEngine::DEFAULT_43_VIDEO_HEIGHT;
   mState = kAllocated;
   return NS_OK;
 }
 
 nsresult
@@ -403,16 +409,22 @@ MediaEngineDefaultAudioSource::Allocate(
                                         const MediaEnginePrefs &aPrefs,
                                         const nsString& aDeviceId,
                                         const nsACString& aOrigin)
 {
   if (mState != kReleased) {
     return NS_ERROR_FAILURE;
   }
 
+  // Mock failure for automated tests.
+  if (aConstraints.mDeviceId.IsString() &&
+      aConstraints.mDeviceId.GetAsString().EqualsASCII("bad device")) {
+    return NS_ERROR_FAILURE;
+  }
+
   mState = kAllocated;
   // generate sine wave (default 1KHz)
   mSineGenerator = new SineWaveGenerator(AUDIO_RATE,
                                          static_cast<uint32_t>(aPrefs.mFreq ? aPrefs.mFreq : 1000));
   return NS_OK;
 }
 
 nsresult