Melee special attacks, also called "Yellow-damage" melee attacks due to the color of their on-screen damage numbers, are not resolved the same way as white-damage melee attacks. Yellow damage attacks include all instant attacks (such as Overpower and Backstab) and all on-next-swing special attacks (such as Heroic Strike and Cleave).
The exact mechanism by which special attacks are resolved is not yet known. What is known, however, is that a special attack made against a mob or a shield-carrying player can be blocked and be a crit at the same time. This result is impossible under the mutually-exclusive attack results table described in the preceding section.
Two-roll theory for melee special attacks
In a discussion thread at http://elitistjerks.com/showthread.php?t=9104
, a user named Vulajin kept track of his critical hit rate over many many Backstabs.
He corrected for all possible factors he could think of, and in the end discovered that the rate he got was consistent with his attack being resolved using two die rolls: A first roll to determine whether the attack missed (using the assumed miss chance for the mob targets he was attacking), and a second roll to determine whether an attack that didn't miss was a critical hit (using his tooltip Crit chance adjusted for the mob target's level).
If his data are accurate, and if the game mechanics for yellow-damage attacks haven't changed since the time that discussion thread was written, then there are at least two random numbers generated to determine the outcome of special attacks. It is unfortunate, though, that these tests were done with Backstab and not with a special attack that can be made against a mob from the front, e.g. Sinister Strike or Heroic Strike. Such data would be more useful, because attacks from the front can be parried and blocked.