It pops up when there is a HTTP 404 status code or similar returned. So these false negatives are likely due to the specific sites that are configured in a wacky way.
(Don't get me wrong, it is still very annoying for the user regardless what the cause is.)
Does it pop up for any 404 error? If so it might be some script or font or whatever resource the site itself is using that would otherwise fail silently. If not... then there has to be some other bug/issue because i get it for many different sites that shouldn't have it.
(Don't get me wrong, it is still very annoying for the user regardless what the cause is.)