Dungeon collapses after vault guard fails to lead you out.
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 vault. 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().