[Update: Valve has shut down the Steam store.]
MMO Fallout can confirm that Steam has experienced a major client bug, causing players to be able to access other accounts at complete random. Speculation is running rampant on the cause, which some are pointing toward a simple caching error, however it is possible to view random users Steam details, including their wallet, owned games, purchase history, and the last digits of their credit card on file.
We tested it on our own systems and managed to log into multiple accounts, although the service is presently barely functioning and crashing pretty regularly.
The good news is that users cannot do anything with the account, be it make purchases, change passwords, etc. It is also possible to access accounts that are protected by Steam Guard.
MMO Fallout 