In 2010, Google rolled out multiple log-ins. It was a fabulous thing for those who didn't want to run multiple browsers in order to have simultaneous access to Google-powered personal and work accounts. Merging your own accounts is one thing, but can you imagine accidentally merging yours with someone else's? It's kind of a pain in the arse to fix.
A semi-recent posting to the product forums on Google is a helpful reminder to be wary of letting your not-so-savvy friends log into their personal Google accounts on your computer. As Molly Fisher points out, it can be a harrowing and confusing ordeal:
I let someone else borrow my computer and when they logged into google, they managed to merge our accounts. Her email address was the "primary" and mine was the alternate. I removed my email from the account, but that ended up deleting my entire google account. I thought re-signing up with that email address would fix things, but it just created a blank account, so it appears I have lost all of my google+ account info and the things in my google drive (I wasn't using the email). I have some external links to some of my googledocs and they still work, but none of the docs show up in my newly created account. Help! How can I get back my google drive and my google+ account?
This happened to me once, and it took me forever to figure out how to un-merge the unwanted account and restore peace to the land. This is one of those things that you don't ever anticipate happening until it happens. Don't let this happen to you! Always remember to launch an incognito tab if your friend asks to use your computer. Or, better yet, tell them to keep their hands to themselves. [Product Forums via Hacker News]