When Google gets things right, users rejoice. When it doesn't, trying to tell Google about it is like praying to God: you say the words, but you never know if they're listening.
Multiple sign-in is probably one of most bug-ridden pieces in the whole Google infrastructure.
Sometimes it fails to provide a link or simply doesn't react when you click on it when you're trying to sign in into several accounts at the same time (to reproduce: click your name on top right/ switch account/sign in to another account => nothing happens 100% of the time).
Sometimes, it forcibly logs out all accounts logged in, and logs them in into whatever account du jour it feels like.
Sometimes, it throws a login/password screen at you right after you have logged in into a totally different account (or so you thought).
I'm not a paid Google tester and not going to get all worked up collecting use cases for these bugs to send it to them so they can fix them (see above about praying to God), but I thought they have enough testers to take care of the very face of Google (which I believe authentication and authorization is).
Looking at Google's face, all in pimples, makes me nervous.
And just now, it threw a 500 Error at me.
UPDATE: Fresh in: now in some places it doesn't offer to "sign in as a different user", it simply says "sign out and sign in as a different user". This blows *ALL* accounts currently signed in out of the water. Oh, and it is, apparently, their "new sign-in page".