PSA: Steam Issue - Log Out Right Now [RESOLVED] · 9:44pm Dec 25th, 2015
Update 2: The issue is resolved.
Update: Just stay off the Community and Store pages. Those were the affected systems. Also, they're offline now, so the issue is being resolved. It's safe to play games or to download them (if the download servers are still online).
According to this thread on r/Games, Steam's fucked something up, and now various actions (such as attempting to make purchases) result in you being logged into the same page on someone else's account. You will see such personal details as full name and home address.
Or someone else will see yours.
Apparently, you can remove yourself from the pool of accounts which people are randomly logged into just by logging out of Steam (exit the program completely, change user and just don't log in, restart in offline mode - all of these work), which removes you from the server's cache or something, I don't understand the technobabble fully.
Currently, the Steam store is frozen and appears to have been taken offline, so you don't need to worry about potentially losing money to a random unethical jerkwad at this point, but there's still a major privacy concern.
Valve be all:
i.imgur.com/q9dZotD.png
NO, SHUT THEM ALL DOWN!
Awww. Merry Christmas.