After all these years, I might have found a rather severe gap in the rules.
Situation was this: The players hear a bad guy approach. One of them steps forward and tries to put a block on him, hitting him with a nervestrike so he can't move (or sound an alarm).
The bad guy in return puts a grapple on this character. He's got a stunt that allows him to do so immediately, without setting up an aspect first (by having a giant crab's arm with a claw).
The problem that now occurs is this: When you establish a block (including grapples), those rolls aren't opposed. You don't defend against a block, it always gets established and if someone takes an action that could be opposed by the block, you can use the block to oppose it.
But what happens here? Does the nervestrike block the claw grapple? It feels like it should. On the other hand, you don't get a defense roll against a block or a grapple, so you can't substitute the block either.
In the situation above, the nervestrike block was established at +5. The claw grapple was established at +5.
What happens?