Continued from here

Originally posted by Nathan Long:

The Spirit shuts down that player being able to draw once they're drawn a card that turn. And can't effect beat out can effects. Since Naga has already drawn for the turn…

I think I made the situation too complicated, but Naga hadn't drawn for the turn :-)

Appa draws on Draw Step.
Naga Cycles (or, to make the situation one that could happen in a Legacy match cracks Horizon Canopy) on Appa's Draw Step.
Naga would draw, but Appa's Notion Thief tries to replace Naga's draw with a draw for Appa.
Spirit of the Labyrinth prevents Appa from drawing.
Does the draw return to Naga since Appa's replacement event failed? Or does Naga's draw get replace by Appa's “fail to draw,” leaving neither player with a card?

It's perfectly possible for a replacement effect to result in the game trying to carry out an impossible instruction. If this happens, the impossible instruction is simply ignored.

In your example, Notion Thief steps in and change's Horizon Canopy's “Naga draws a card” event into “Appa draws a card.” The game tries to carry out this instruction, but Spirit of the Labyrinth renders it impossible, so nothing happens. Nobody draws a card.