It seems it doesn't matter how big you are, it's the simple over-sights that catch you out. Logging on to my adsense account this evening I was presented with this security window warning me the certificate had expired. Looking at the details, it expired at 19:52:39 BST.
They may be worth billions, but they need to spend a few extra quid on someone to do the admin. I've emailed them to let them know; let's see if they reply.