plover writes "Kim Zetter of Wired documents an extensive hack of Wal-Mart that took place in 2005-2006. She goes into great detail about the investigation and what the investigators found, including that the hackers made copies of their point-of-sale source code, and that they ran l0phtCrack on a Wal-Mart server. 'Wal-Mart uncovered the breach in November 2006, after a fortuitous server crash led administrators to a password-cracking tool that had been surreptitiously installed on one of its servers. Wal-Mart's initial probe traced the intrusion to a compromised VPN account, and from there to a computer in Minsk, Belarus.' Wal-mart has long since fixed the flaws that allowed the compromise, and confirmed that no customer data was lost in the hack — which is why they did not need to report the breach publicly earlier."
This intrusion happened around the same time that Albert Gonzalez's gang was breaking into Marshall's
and its parent company, TJX. The MO was quite similar: researching and closely targeting the point-of-sale systems in use. But the article notes that "There's no evidence Wired.com has seen linking Gonzalez to the Wal-Mart breach."