Back in late January 2009, Blizzard's advertisements were accidentally swapped out with Gold Seller ads. (Links: wow.com, cgenetwork.com, 1up.com)
Other than this one incident, I recall that it happened a few times actually: on BNet Forums, WoW Forums, and the waiting room of Diablo 2.
But in each case, most (if not all) news instances started to "disappear" and people later forgot. It didn't help though that threads were locked, forum entries were deleted, and Blizzard told sites to remove the news either.
So it's difficult finding information on these accidents now and it's something that Blizzard would prefer that you forget. =]
Reading over the forums, you'll see that Blizzard employees and players alike will all insist that "Blizzard has NEVER been breached. They've never been hacked. They have security measures in place that make them foolproof and 100% protected."
(Note: I never understood that logic so I simply chalk it up to "Blizzard Indoctrination". It's funny because if a game developer creates games that melt video cards and are full of bugs, cheats, issues that take years to fix, exploits, and can easily be hacked and exploited.. why would anything else they create be so different?)
Although employees insist that "To this date Blizzard has never been breached", in an earlier post I listed many cases of Blizzard database breaches, how Blizzard employee accounts have been compromised (and used to spread keyloggers on official forums), and other hacking activities. When the Blizzard employee accounts were compromised, Blizzard sent notifications to websites to remove the information ("Reputation Management"). And those were just the breaches that made public headlines! Who really knows how many breaches there have been?
Some of these are just accidents and "glitches" while others are major security breaches.
This has been going on for years, but it's obvious that it has been forgotten by most. That's why preservation is so important. =]