Special Terrain such as Deadpool's Merc Jet or the Invisible Plane states the terrain is hindering terrain for line of fire purposes and blocking terrain for movement and occupancy purposes.

Knockback's third trigger says:
Knockback places instead of moves, but would the special terrain trigger knockback damage from shortening a knockback as it being blocking terrain for occupancy purposes would mean the square is "occupied" by blocking terrain?

Yes, the knock back path would be stopped due to not being able to place the character in a square of blocking terrain, so it would cause knock back damage.

That was my take as well, I think where the fellow I was debating with (hi, pupulesurfer!) is getting hung up is on when it operating as blocking terrain is triggered on occupancy.

I hesitate only in that the check asks if the next square is blocking. It isn't, it is special which has its own definition. For knockback, it isn't blocking.

You then try to occupy it but can't because now it is blocking. I would then argue the rule of occupancy comes in. Where am I missing it or am I on the right track?

The fact that you can't place a character in the square, which is considered blocking terrain for occupancy, is what would end the knock back. The check is "I'm going to place a character in this square, is it blocking terrain?". The answer is "yes, because you're trying to occupy this square".

You're welcome to ask on the WIN. I haven't actually seen this come up before. I somehow doubt that WK's is going to rule that you get to knock a character back and then get to use the Rule of Occupancy to then place the character where you wanted.

But if the knockback distance would place the character beyond the Invisible Plane, they would be placed normally, yes?

Knockback places one square at a time, not the full distance, so the first square would try to be placed in and cause an occupancy issue.

Makes sense.

Thanks.

I think technically this comes down to the golden rule of occupancy, and since knockback is generally optional, if the knockback would cause the character to 'enter' the special terrain, you can't choose to use knockback.

Wouldn't this trigger on checking knockback before anything about double occupancy?:
Once again, Special Terrain says that it is blocking for occupancy. I thought it was more complex at first, but because the check is specifically blocking terrain occupies that next square it seems like it would trigger knockback damage like any other blocking terrain.

Not sure if I understand you right, but I don't think having special terrain a couple squares behind a character you could otherwise knockback prevents you from using knockback. You also don't get to choose square by square, so you can't get to just in front of the special terrain and then not be able to choose further knockback.

I think it is just that the knockback sequence says it stops when "blocking terrain occupies the next square or is just before the next square." The special terrain is blocking for occupancy purposes, so I think it occupies the next square and you stop right before, and deal one knock back damage.

Blocking terrain occupies squares. If the knockback sequence checking for blocking terrain occupying the next square would find blocking terrain when testing this Special Terrain, then that would be equivalent to any effect looking for blocking terrain finding it, which specifically violates the quoted text about mixed terrain. When the special terrain says it is blocking for occupancy purposes, it is referring only to purposes involving other characters/objects/markers occupying those squares.

'Technically speaking', if this terrain is 2 squares behind a character I knocked back, and they would be knocked back 1 square, then that's fine, but if they would be knocked back 2 or more squares, I am now presented with an optional effect that would lead to a violation of the golden rule, which by definition means I can choose to activate it. On the other hand, if I used Force Blast, and happened to roll high enough, it is no longer optional, and 'technically' you(the opponent) would pick one of the nearest unoccupied squares, and continue the sequence. However, this is a lacking of the knockback sequence to check for such a condition. A good intent ruling would probably indicate to do what Lantern previously stated.

By this rationale you can never knockback a character into blocking terrain, because knockback is optional 99% of the time and always places.

The rule of occupancy has no bearing here (beyond preventing a character from occupying that square).

Not really. The rules of the knockback sequence specifically check for blocking terrain in the next square as one of the stopping triggers. No 'illegal game state' ever exists, or tries to exist. The special terrain technically(probably should be an intent ruling here) does not trigger condition 3 of the sequence, or any of the other conditions, therefore, within the context of a knockback sequence already started, you must non-optionally place the character in the next square, where the special terrain now acts as blocking, preventing this step. This would be why, if you 'know' they will end up trying to be placed in the special terrain, you 'technically' couldn't choose knockback, while if you don't 'know', and started a legal effect that after that point causes non-optional knockback, it would invoke the rule of occupancy. This should 'probably' just be ruled that step 3 of the knockback sequence also checks for terrain that is blocking for occupancy purposes.

It's becoming clear that there is some semantic jujitsu that needs to be done to make "occupancy" work the way we think it should. For example, objects do not "occupy" squares:

But walls "occupy" the edge of the squares they touch:

Game elements are defined to occupy their squares:

Special terrain is a game element, as defined on page 34. So at least the special terrain occupies its own squares. I can't prove that printed squares of blocking terrain "occupy" squares, but that's beside the point for the purposes of knock back through special terrain.

When knock back checks if a square of special terrain is occupied, the answer is yes. For the special terrain being discussed, when you check for occupancy the terrain type is blocking. You don't need to attempt to place a character into the terrain for the occupancy check to occur. By nature of being special terrain, the square is "occupied" and counts as hindering when knock back asks what kind of terrain occupies that square.