A476

An AoO can prevent the completion of the provoking action in the following circumstances:
1) The provoking action is spellcasting.
2) The result of the AoO makes the provoking action impossible, independent of it being an AoO.
3) The character making the AoO has a special ability that makes the AoO prevent the action.

In case 1, the damage from the AoO counts as being during the casting and therefore forces a concentration check.

Case 2 can result from situations such as tripping someone who attempted to move or disarming someone who attempted to use a weapon in a way that provokes. A successful trip AoO would make the provoker prone, and being prone prevents normal movement. A successful disarm, in response to someone attempting a sunder without the Improved Sunder feat for example, makes any form of attack with the weapon in question impossible. Dealing enough damage to drop the provoker to either death or unconsciousness also prevents any action he was attempting.

An example of case 3 is the feat Stand Still.