Part 6 - Sequence break
The out-of-bounds mountains are northwest of the no longer icy area. "You somehow manage to get onto some of the mountains. However, they are very slippery, and seem to be trying to push you back to the southeast. You notice some of the parts of the mountains you couldn't see are missing, and there's a big black void there." Instead of doing something when the player is in the out-of-bounds mountains, now the player is in the no longer icy area.
Instead of going north when the player is in the out-of-bounds mountains:
say "You try going north only to find yourself falling into the black void.";
fall.
Instead of jumping when the player is in the out-of-bounds mountains:
say "You jump in place at the right time, preserving your momentum. However, you end up falling into a pit.";
now the player is in the pit itself.