To reproduce: 1. Go to a page with <video controls> 2. Click the fullscreen button in the video's controls 3. Switch to another window via Alt-Tab (e.g., the Safari window that contained the <video>) 4. Look in the taskbar Another taskbar button has appeared. It has no title but has the Safari icon. Presumably it represents the fullscreen video's window.
<rdar://problem/7547576>
Jer says this was fixed in r60190 along with bug 39557.