Well it happend to me today, I accidently deleted the Error page 403!
It says inherited, so I tried to find how I can inherite from the top site, no luck. I even removed HTTP Errors and re-added...no luck
I manually created 403, its "Entry Type" was set "local" and I got 500 error when I try to open the site.
Finally I looked at the web.config
howa,,,there it is
I removed it, and refreshed the iis...yahoo
I got my 403 back!
That was dumb, but I am glad it is fixed!