On Christmas Day, Steam had quite a big hiccup. Randomly players began to see information of other players as if they were logged in as them. It got out this was due to a cacheing error, but Valve remained quiet.
Well, now five days later, Valve have finally spoken up about the incident. While they reiterated no serious action could have been taken and users were safe, they explained the whole issue started with a DDOS attack:
Early Christmas morning (Pacific Standard Time), the Steam Store was the target of a DoS attack which prevented the serving of store pages to users. Attacks against the Steam Store, and Steam in general, are a regular occurrence that Valve handles both directly and with the help of partner companies, and typically do not impact Steam users. During the Christmas attack, traffic to the Steam store increased 2000% over the average traffic during the Steam Sale.
In response to this specific attack, caching rules managed by a Steam web caching partner were deployed in order to both minimize the impact on Steam Store servers and continue to route legitimate user traffic. During the second wave of this attack, a second caching configuration was deployed that incorrectly cached web traffic for authenticated users. This configuration error resulted in some users seeing Steam Store responses which were generated for other users. Incorrect Store responses varied from users seeing the front page of the Store displayed in the wrong language, to seeing the account page of another user.
It's good to know that is seems no serious damage took place, but I'd still have liked this explanation much prompter. Still, no harm no foul I suppose in my book.