--- a/content/smil/nsSMILAnimationController.cpp
+++ b/content/smil/nsSMILAnimationController.cpp
@@ -353,16 +353,20 @@ nsSMILAnimationController::DoSample()
 
 void
 nsSMILAnimationController::DoSample(bool aSkipUnchangedContainers)
 {
   if (!mDocument) {
     NS_ERROR("Shouldn't be sampling after document has disconnected");
     return;
   }
+  if (mRunningSample) {
+    NS_ERROR("Shouldn't be recursively sampling");
+    return;
+  }
 
   mResampleNeeded = false;
   // Set running sample flag -- do this before flushing styles so that when we
   // flush styles we don't end up requesting extra samples
   mRunningSample = true;
   nsCOMPtr<nsIDocument> kungFuDeathGrip(mDocument);  // keeps 'this' alive too
   mDocument->FlushPendingNotifications(Flush_Style);