Nov 15th 2010 1:48PM @tamadams23:
Much like the FD bug, the Vanish bug had been around since the beginning of time. It got partially fixed with Wrath (not sure on the exact patch), and is now fully fixed with 4.0.1. Essentially, when a rogue would use vanish, it wouldn't always work. If a rogue was hit at nearly the same time they used vanish, it would instantly break the vanish/stealth and the rogue would most likely end up dead. This was a problem in both PvE and PvP, but in PvE it was even worse since mobs tend to stick on someone for a second or so after being removed from combat.
The fix in wrath addressed this by forcing stealth to persist for something like 0.5 seconds after vanishing (no damage would break stealth for this time). It worked for the most part, but would occasionally result the same way as the old bug. They increased this to 3 seconds in 4.0.1, which effectively ended the reign of terror this bug held.