I believe that these are two side-effects of this parent bug:

(1) Now that there is a final document character that is locked alongside "E N D
  O F   D E L E T I O N S", which is also locked, typing anywhere in that chunk
of locked text causes a "IndexError: list index out of range" error (as per
msg84). This is because typing in locked text requires scanning for the
beginning and ending of that text. The scan to the end causes the crash via the
run length encoding error.

(2) Attempting to repeatedly creep right from the final document character (try
10 creeps in a row) permanently slows all of Archy down until Archy is
restarted. CPU usage during any cursor movement goes from sipping 5-7% before
the slowdown to 100% after the slowdown. I am assuming this is related to the
run length encoding bug.