> For example, "That's the most dumbest thing I've heard!" (when only "dumbest" is needed), or "This shirt's more prettier than that one" (instead of just "prettier")...

Some time ago, I overheard two cafeteria workers discussing the then new movie "Anaconda." One asked how the other liked it. The other said it was ok and added, "It shoulda been more bloodier."

Bad English with bad movie criticism in one shot. :-)