--- a/dom/canvas/TexUnpackBlob.cpp
+++ b/dom/canvas/TexUnpackBlob.cpp
@@ -307,16 +307,20 @@ TexUnpackImage::TexOrSubImage(bool isSub
                                                       dstOrigin))
         {
             break;
         }
 
         return; // Blitting was successful, so we're done!
     } while (false);
 
+    webgl->GenerateWarning("%s: Failed to hit GPU-copy fast-path. Falling back to CPU"
+                           " upload.",
+                           funcName);
+
     RefPtr<SourceSurface> surface = mImage->GetAsSourceSurface();
     if (!surface) {
         *out_glError = LOCAL_GL_OUT_OF_MEMORY;
         return;
     }
 
     TexUnpackSurface surfBlob(surface, mIsAlphaPremult);