The glitch is believed to affect Google Webmaster Tools, a web service that lets developers and administrators check the indexing status and optimise the visibility of their websites.
The problem has been reported by SEO specialists and others, throwing some site administrators into a panic.
It means some websites are open to unauthorised users, potentially exposing them to the risk of sabotage and misuse.
The problem was highlighted by one Google Webmaster Tools operator, SEO blogger David Naylor.
He said his company had "regained access to every old account we have previously been given access to, whether that is a previous client or maybe a site that came to us for some short term consultancy".
Mr Naylor explained: "There is a serious risk that damage could be caused to sites by people who no longer have permission to make changes.
"Things like disavow link lists, de-index URLs or the entire site, redirect URLs, geolocation alterations ... a whole world of pain."
Many Twitter users have commented on the apparent glitch, but there has been no immediate comment from Google.
Google Analytics, a service that generates detailed statistics about the visits to a website, is also thought to have been affected by the problem.
This means further confidential data could be accessible by non-verified users.