@gibson042, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jaubourg, @markelog and @adamcoulombe to be potential reviewers.

I can't find any documentation that would explain why callbacks.add(fn) on a locked callbacks object is supposed to run the function. Is the documentation lacking or am I just not seeing it?

memory: Keeps track of previous values and will call any callback added after the list has been fired right away with the latest "memorized" values (like a Deferred). https://api.jquery.com/jQuery.Callbacks/

If the Callbacks object is created with the "memory" flag as its argument, additional functions may be added and fired after the callback list is locked. https://api.jquery.com/callbacks.lock/

Not sure if that makes total sense to me, but those are the docs.

Right, it's actually the difference between .lock() and .disable().

Yeah, mainly trying to think of specific use cases for all the Callbacks options. Clearly we haven't even probed them all for core. :)

This broke an effects test; I'm investigating now. http://swarm.jquery.org/job/4519