Steam has been pretty borked today. When I go to log in, it shows me other people's details. I nearly crapped myself when I noticed two items in the cart that I hadn't put there. I swear I went into full-on OMGHACK mode, but it turns out to be not as bad as it first seems.
(update 23:49GMT 25.12.15: apparently it's all been fixed now)
From what I can gather, this isn't a hack. There's a problem with caching, so the servers basically show you the page that someone else clicked on, and that person might get someone else's page and so on. It looks like you can't actually do things like buy games with other people's credit or payment details.
What's probably more of a worry is that other details can be seen, like email addresses, last 4 digits of credit card numbers and so on. Were I a nefarious type, rather than the lovely chap I am, I would have been writing a script to refresh the accounts page (as you get different users) and hoover up all those details.
Anyway, it looks like the best thing you can do for now is to just avoid using Steam until the caching issue is fixed. Because the page you request can be served up to someone else, its probably best NOT to go changing credit card or other payment details from within Steam now. If you have a Paypal account linked to Steam, you can unlink it from within Paypal, though.
SteamDB (NOT affiliated with Valve)
Reddit (not particularly informative, but if you're a sadist then it's kinda fun to see everyone losing their shit)
Steam Thread (probably the most 'official' announcement, plus more people losing their shit in the comments)