The answer is, "when it's the same goddamn Tollbooth that you've had to hit three times to get into this blasted server because of Mumbad City Grid."
Let's be very clear about what happens when you have Mumbad City Grid on a server:
- The runner approaches the outermost piece of ice. The corp chooses to rez or not as normal. If they rez, the runner encounters the ice (triggering "on encounter" effects like Tollbooth, Data Raven, Quicksand, or Komainu). If an "end the run" effect happens during this encounter, the run is over and Mumbad City Grid does not fire.
- If the corp doesn't rez, OR if the runner breaks the subroutines, OR if the subroutines fire but don't end the run, the runner has now passed a piece of ice. The Grid fires: the corp must immediately decide whether or not to swap the ice. The rezzed/unrezzed state of both pieces of ice is irrelevant. (This is also the trigger point for The Twins, provided the ice they just passed was rezzed.)
- The runner may now jack out or continue the run, from their position one ice deep into the server. Swapping ice does not move their location.
- If they continue the run, and there is another piece of ice, repeat.
What this means is that if you have a server with two unrezzed pieces of ice, and a rezzed Tollbooth on the outside, you can do this: runner encounters Tollbooth, breaks it, swap with next piece of ice, runner hits Tollbooth again, breaks it, swap with last piece of ice, runner hits Tollbooth a third time, breaks it, and you put the Tollbooth back on the outside of the server.
That's just cruel. Just pure meanness. A triple-Tollbooth server is the closest thing to impenetrable the game currently has, and you don't even need to rez more than one. A triple-Komainu can easily get up to 10+ subroutines before they make it in--remember, it's the same piece of ICE being encountered again during the same run. Quicksand accumulates counters at triple the normal rate.
Of course, if the runner breaks the Tollbooth once and jacks out, you now have the Tollbooth as the middle piece of ice; then, the next time they run the server, they only have to deal with Tollbooth twice (plus the chump piece of ice on the outside). Unless, of course, you happen to be running Tenma Line, a criminally undervalued card.
Mumbad City Grid is the first piece towards a new kind of Jinteki glacier; when the economic ID of Pālanā Foods comes out in the next pack, we might see a rival a to the legacy of Replicating Perfection arrive in the meta.