Answer: When it was allowed as a goal! Update: Someone at YouTube has obviously been very busy after a call from the Football League's legal department! You can see the "goal" here as long as you on a UK IP address - right hand box - scroll down to choose "20 Sep 08 Watford 2-2 Reading". The red arrow below points out the video box on the page. The blue arrow points out the scroll down menu to find the Watford v Reading match.
Just to be on the safe side, the link to the page containing the video is also here: