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C343-23 is a core bug. It is listed as:

Dungeon collapses after vault guard fails to lead you out.

It is currently listed as "Open" on the official bugs list, but for a long time was listed as "Help" as its cause was unknown; however, in 2012, Grunt was able to reproduce it:

The straightforward way to reproduce this is as follows: 

1) Teleport into a vault.
2) Wait for the guard to start leading you out, so that you can observe
   the path he takes. (The bug isn't necessarily reproduced if the guard
   doesn't take a straight path for the first two steps, as this yields
   more than one accessible tile from his stopping point.)
3) After the guard disappears (once he starts leading you out, you can
   just teleport elsewhere and he'll disappear), teleport back into the
4) Dig out the first two steps of the guard's path after the vault wall
   (such that there are two corridor tiles).
5) Return to the vault and wait for the guard to lead you out again. 

Result: the guard gets stuck on the last tile dug out. The code that
extends the corridor believes he's still moving, however, and eventually
results in an overflow once the length of the internal fake corridor
buffer is reached, causing a panic().

A ttyrec of the bug in action is available here: http://dl.dropbox.com/u/5494313/c343-23.ttyrec