First - something, that I guess should is an error in the rules. The Stacking rules on p. 57 TW say to check for stacking limit violations at the end of each Movement Phase. The Stacking rules in the Introductory Rulebook (p. 27) don't define an exact moment, but seem to suggest to do it right after any unit ends it's movement doing Movement Phase. The way, that Displacement and Domino Effect rules are phrased would suggest one of two things:
- you should check stacking limits every time a unit ends it's movement OR IS DISPLACED,
- you should check stacking limits at the end of EACH Phase (not just Movement Phase).
This probably should be clarified by an errata.
I've got a bigger problem though. I can't figure out what should be the sequence of the following events:
1. resolving unit displacement and movement due to physical attacks/domino effects (of both attacker and target),
2. resolving falls due to physical attacks,
3. applying damage from charge, DFA and falls,
4. checking stacking limit violations, and resolving domino effects,
5. resolving unit destruction (including removing unit from the board) (p. 128 TW and p. 49 Intro Rulebook),
6. checking if a unit causes collapse of a building hex, entire building or a basement (pp. 176 - 179 TW).
There are several possible situations when this sequence of events is important, for example:
- If 5 happens before 4, than a unit destroyed directly or indirectly (for example by a fall) by a physical attack or domino effect, which would also cause the unit to be displaced, than the destroyed unit can't cause or propagate a domino effect or damage/cause a collapse of a building it would be displaced into. When playing with TO the sequence of 5 and 4 may also influence where engine and ammunition explosions (p. 77-78 TO) happen.
- The sequence of 1 and 3 may be important if being displaced may cause the displaced unit or something else something to be damaged. This happens if a unit is displaced into a building for example. Consider:
What do you mean by taking normal damage from entering a building hex, before resolving the charge? Don't I need to at least make a to-hit roll for the charge I don't even know if I'll end up in the hex in question (see Location After Attack on p. 148 TW)?
Is the sequence:
- make to-hit roll for the charge,
- if you succeed (meaning you will enter the building) resolve the entire procedure from pp. 167-168 TW (piloting roll and possible damage for entering a building hex),
- resolve charge damage,
or is it something else?
If 3 happens before 1, then you should resolve charge damage before entering the building and following the procedure from pp. 167-168 (PSR to avoid damage due to entering a building) TW.
- If 5 also happens before 1, then in the example above if a unit charging a building destroys itself by a charge damage to attacker it gets removed from the game before a PSR to check for damage due to entering a building can happen.
- The sequence of 2 and 3 can also be important in the example above, as it may determine, if a unit charging a building may take falling damage before it enters the building (which may also cause damage to both the building and the unit).
- Depending on the final ruling about the damage to a unit displaced to a building hex, the sequence of 2 and 3 may be also important if a unit falls and gets displaced into a building hex by a charge/push/DFA/domino effect etc.
- Depending on the sequence of 5 and 6 (and other events, that may lead to 5) a unit may get destroyed and removed from the board before it's weigh may cause a building or basement to collapse.
And so on... You could probably invent plenty of other situations, where a sequence of those events may influence final outcome.
When deciding when 5 happens (destroyed unit gets removed from the board), you should also consider what happens if you play with TO rules, that allow to carry or drag a destroyed 'Mech (pp. 92 - 99 TO), in which case it needs to stay on the board even after it is destroyed.
You should also consider, that the precise moment a unit's destruction is resolved may influence damage that it will take for the purposes of salvaging/repairing it (for example - does such unit take damage from being displaced into a building by a physical attack, if such attack would deal enough damage to destroy the unit). I seem to recall the a rule, that if a 'Mech is destroyed, you track damage to it until the end of a current phase, but I can't find it at the moment. I'll add an appropriate page reference if I find it. The closest thing I can find at the moment are the rules on repair and replacement and salvage in the pp. 175 - 192 SO, but those are not quite the rules I mention here - they only say what you can do with a destroyed unit after the battle depending on damage it suffered.
Another problem to consider is if a destroyed unit may contribute its mass to a collapse of a building or a basement. It may be relevant during the phase the unit was destroyed depending on sequence of 5 and 6, but may also be relevant during further phases if we track position of destroyed units for the purposes of TO rules about carrying or dragging destroyed units mentioned above.
EDIT - I realise, that some of the particular situations, I've given as examples above may have been addressed in the previous rulings on this forum, but I'm afraid, that as long as we don't have a unified sequence, new questions may keep popping up due to general ambiguity of the sequence of events listed, and multiple ways some of them may be triggered together. Plus - without one general rule on how to establish sequence of such "simultaneous" events, you run a risk of creating rulings for particular situations, that contradict each other.