When you add a site to Google Webmaster Tools, it asks you to verify it (e.g. via a <meta> tag).

I verified a site a while ago, but when I logged in, I noticed that it isn't verified anymore. The history shows that it was verified 58 days ago, but then 30 days ago it tried and failed saying that "revierification failed".

I'm not sure if this is a result of some setting I changed which required a reverification, or if Google Webmaster Tools periodically tries to verify the site.

I was under the impression that the verification only happens once when you add the site, and then you can delete the <meta> tag. If this is not how it works, and it does reverify periodically, will it require a different <meta> tag value or can I keep the original one I used and never have to worry about it again?