Patchlevel 9 is in fact an amendment patch to Patchlevel 8.  It
contains fixes to some deadly bugs discovered thus far in the Pl8
code.  

Among the bugs fixed are a 0 dereference bug in the Amiga code, a
bug in the tins code, a bug in the inventory weight code, a level
files bug in the Mac code, an ST display bug, a bug in the maze's
bones level, a bug with chest traps, a bug in the restoring code, a
vault guard bug, a PC collapsing dungeon bug, and other more minor
bugs.