A Google Webmaster Help thread has a report that someone noticed their site was verified in Google Webmaster Tools without them actually clicking the “verify” button. How did this happen?

Well, first of all, the www version of the site was verified. The non-www version they did not manually click verify for. So, Googler, Dennis G. explained in the thread:

Pretty neat, at least I think so.

Forum discussion at Google Webmaster Help.