This question dates back to when Wall of Tanglecord was in the current draftformat and the attacker had flying as a static ability. Back then, you were already in declare blockers and it was to late to give the wall reach. Lets assume that there are multiple attackers and multiple blocking ongoing. With todays rules regarding out-of-order sequencing you would be ok:

MTR 4.3
…
5. A player declares a blocker, animates a Treetop Village, and then attempts to block with that Treetop Village.

So, you would be ok today to block say 3 attackers with your 3 blockers and then animate the walls reachability.

__
Now it gets quite a bit more tricky. What if the block is not (knowingly) illegal until it is performed? This is of course a case of missed triggers. The scenario is as follows:

AP has a Weldfast Wingsmith and plays an artefact spell in first main phase. AP does not announce the trigger - As we all know, it is not missed, but neither is it remembered at this point. AP then attacks with Wingsmith.

NAP has Wall of Tanglecord in play. NAP wants to block Wingsmith, at which point AP points out it has flying. Again, there are also other creatures attacking and blocking.

Important!
NAP does not activate the wall in a timely fashion for it to be a clear out-of-sequence ordering. On the contrary, NAP has good use of the spared mana should he not need to spend it to activate the wall.

Question
The block is clearly illegal, and the declare blockers will need to be done from scratch. NAP has however not done anything wrong since the triggers was not announced; the block was legal until AP stated it was not.

Will NAP be able to activate the walls ability to give it reach? Or does this fall under the “if you need to know you have to ask” section?


As always, AP may of course act as if the sequence had been carried out correctly. In the Treetop village example for instance, AP of course would have the possibility to destroy the Treetop Village with a Ghost Quarter between activation and the actual blocking. Any such what-ifs scenarios would of course be handled by doing things in a correct sequence, so no need to cover those in the reply.

In my opinión, this falls under the “if you need to know you have to ask” section
For it to be OOOS, the player needs to be doing a single block of actions, without gaining any extra information
In this case, if you have to verify the legality of your declaration, or the opponent tells you that the blocks are not legal, its not a single block of actions anymore. So, “if you need to know, you have to ask”

More generally than “if you need to know you have to ask”, the important bit in the Missed Trigger policy to point out to the Wall of Tanglecord player is “Triggers are assumed remembered until otherwise indicated.”

The player challenged the trigger by attempting to block with a creature without Reach - but by doing so forfeited their opportunity to modify the blocking rules. I don't think OOOS can reasonably apply, because this very act is literally game knowledge gained that would otherwise affect the actions taken.

Edited Rebecca Lawrence (April 27, 2017 02:44:48 PM)