Lion Preview window stuck as always on top After accidentally pressing some wrong keys, it seems my Preview window is stuck in 'Always on Top' mode.  I can't figure out how to undo this.  Is anyone aware of what might have happened?

edit: Well I just closed the document and re-opened it, and it seems to be behaving normally now.  Still curious as to what happened though.
This happens to me in Lion as well, but only with big PDFs. Closing and reopening fixes it.