For example, a character starts a full attack, and the first attack provokes, and the character takes damage, such that the total is equal to their hit points (they are now at 0 and disabled, or staggered if some of the damage was nonlethal). What further actions can they take? Is the character out of actions, as the first attack now counts as a standard action, or does the staggered or disabled condition only impact actions at the start of your turn? What if it was the second attack that provoked?
Limited actions on my turn: If an AOO or other interrupting effect reduces what actions I can take on my turn, does this reduction apply immediately?
Yes, even if it interrupts or limits your in-progress.
For example, if you are making a full attack and attempt to trip your opponent, but you provoke an AOO because you don't have Improved Trip, and your opponent has a spell storing weapon that's storing a hold person, and you fail your save against the spell, you are immediately paralyzed and can't take any of your remaining actions (including the remainder of your full attack).
Likewise, if your opponent had the Staggering Critical feat instead of a spell storing weapon and the attack staggered you, you would immediately gain the staggered condition, which would prevent you from taking any actions that violate the staggered condition's limitations. If you provoked by taking a move action to move through the opponent's threatened area, you could finish that move action but could not also take a standard action after it. If you provoked as part of a full attack (as with the trip example), becoming staggered would end your full attack at that point and prevent you from taking a move action after the staggering attack. It doesn't matter if the AOO happened because of your first attack in your full attack or your last allowed one, being staggered ends your full attack at that point because you can't make a full attack if you're staggered.
The same logic would apply to becoming disabled or staggered from damage (rather than having the condition arbitrarily applied): You can finish the action (moving, making the one attack), but then have to immediately end your turn (since you have no actions left), even if you're in the middle of a full-attack.
From the PHBv1 for 3.5e
Attacks of opportunity
An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).
After your first attack, you can decide to take a move action instead of making your remaining attacks, depending on how the first attack turns out. If you’ve already taken a 5-foot step, you can’t use your move action to move any distance, but you could still use a different kind of move action.
So in your example, the character makes a standard attack and is disabled, they get to take one move or one standard action for their turn, which is their attack (completing the current turn). They cannot follow up with a full attack as they are disabled and only allowed 1 action per turn and they fall unconscious after making that attack.
If the character was brought below 0 hp from an attack of opportunity, they would lose all remaining actions and their turn would end.
A kind DM might let you change your action if you were disabled, but by RAW you should have to follow through on the attack that provoked the attack of opportunity as that is the continuation of your turn.
Being disabled or staggered does not immediately stop you from taking actions, PHB 3.5 page 145:
DISABLED (0 HIT POINTS)
When your current hit points drop to exactly 0, you’re disabled. You’re not unconscious, but you’re close to it. You can only take a single move or standard action each turn (but not both, nor can you take full-round actions). You can take move actions without further injuring yourself, but if you perform any standard action (or any other action the DM deems as strenuous, including some free actions such as casting a quickened spell) you take 1 point of damage after the completing the act. Unless your activity increased your hit points, you are now at –1 hit points, and you’re dying.
(Pathfinder has a text very/completely similar to this.)
What happens now is up to the DM, since such a specific event is not described in the player handbook. I would say that you have these options:
- Continue the full round attack, you will go to unconscious/dying after the complete attack.
- Continue the attack, you will go to unconscious/dying after the first swing.
- Don't continue the attack, this was no action and you have your entire round left. You are not unconscious/dying yet.
- Don't continue the attack, this was a standard action (to prepare the attack that you cancelled). You have a movement action left and you are not unconscious/dying yet.
- Don't continue the attack, this was a full round action (to prepare the full attack that you cancelled). You have no action left and you are not unconscious/dying yet.
Of all these I favor the second and forth option. I did not take free/swift actions in account for this list.