"FATAL 1: my bits moved right off the end of the world!"

I have run into this error message a few times now in various circumstances
(using pgsql v6.5.2 on Linux 2.2.5-15smp):

    "FATAL 1:  my bits moved right off the end of the world!"

Does anyone have a clue as to what scenarios cause this?  (It apparently comes
from line 506 of .../src/backend/access/nbtree/nbtpage.c)

I've had the luxury of being early in development and being able to rebuild my
DB to remove the symptom, but that will soon change.

Thanks in advance,
Ed Loehr