This is seen when an embedded youtube video comes onto the display when viewing a forum thread:
http://cl.ly/6ZgF

The video is displayed in the top left of the browser. However, when the embed area is not on the viewable estate, it disappears.
This is on Safari 5.1 (7534.31.2) on Lion DP2 Update 2.

Just a heads up in case you guys or Beast haven’t realised.