The user appears to be very busy looking at the forums and gets the 403 error every so often. I don't know who exactly the user is since they haven't complained but the amount of these errors has me concerned. Also I wonder if seeing so many calls for this file in a row is normal.
While I'm at it I am also seeing _403 errors for blank.gif. I haven't investigated this yet but once again the file will return when called normally.