Vyvyan Basterd
Adventurer
There is a difference between a single attack action and an action that includes movement of many squares. There is such a distinct difference between the two that the immediate reaction rules explicitly call out the ability to react to the movement of individual squares. It's pretty clear that movement is an action that has to be dissected into individual components where interrupting only interrupts the last square of movement. Many other actions, though, are nearly instantaneous and make sense as being interruptable for the entire action.
Although amusing, your example here does not negate the overall event.
The movement action did NOT cause the damage. The movement action completely finished at the top of the cliff. The falling caused the damage. It is a different event and not part of the movement action, hence, the PC heals the damage of the hitting the ground event that he interrupted and it does not affect the previous movement action that ended at the moment the PC went over the cliff.
Awesome try though.
First, I think you are choosing to apply common sense when the rules make something ridiculous in one instance, but demanding that RAW must be adhered to in other instances where common sense says something else entirely was intended by the designers.
Second, if you wish to argue this by RAW, show me where in the rules Hitting the Ground causes damage. I can find rules for Falling Damage. Bear's Endurance would then interrupt the Fall. Rewinding to the point before you Fall means that the interrupt occurs when you are still at the top of the ledge. If you are one more square along in your movement, you are Falling. That's not before the damage-causing event, that's in the middle of it. And since you would gain hit points before the fall, you will have already moved the maximum number of squares allowed and be unable to move over the cliff edge.
Now, if you'd like to concede that an interrupt can occur during anywhere in the middle of it, before it resolves, then I will concede your point about falling. But then Bear's Endurance should be able to trigger by RAW after one takes damage, but before they fall unconscious.