Here is a better explanation. I just sent this feedback to Apple:
When previewing a song, clicking play, but then clicking on the stop doesn't actually stop the preview of a song. I'm previewing songs in the iTunes Music Store, where the "Top Charts" are listed on the right. The song keeps playing, the button changes back to the play button from the stop button, but the song keeps playing. Then, I click the button again, and it just restarts the song. There is no way to stop the preview manually. This is happening on 2 computers for me. An early 2008 Mac Pro and an early 2008 MacBook Pro, both running Snow Leopard and iTunes 9.