Notes:

An error, possibly linked with DDOS attacks on steam, resulted in multiple caching failures. As a result, some users were randomly kicked into complete stranger's accounts. It's suspected that only people who were currently online were affected, and many were advised not to log in. It has been fixed, but some questions still linger, contact your bank/credit cards if you're nervous.
Sauce: (forbes had one of the best articles on this issue)
http://www.forbes.com/sites/insertcoin/2015/12/25/steam-is-randomly-logging-users-into-other-peoples-accounts-and-exposing-their-information/