From time to time (2x per year?), Google drops an interstitial after login to verify that it has the correct email address and mobile phone number on file.

While financial institutions are far more likely to have current contact info on their online banking customers, it’s still advisable to check in annually to verify contact info, especially with the growing importance of mobile.

And while you are at it, ask customers for alternate email address(es) and phone number(s).

Google interstitial page displayed after logging in to Gmail (7 March 2011)