I was watching some videos earlier and the Search Bar does not show a thumbnail of the frame when you are sliding it. Has anyone else experienced it, or is there something going on with my Mac? I have been getting that stupid img.mediaplex pop-up lately.

I don't get the img.mediaplex thing you are referring to, but this happens quite a lot with me.
Don't know why, but it just happens. Often refreshing the page solves it, or restarting the browser. I generally leave it though as it doesn't bother me too much.