This article is from the GNU Chess and XBoard FAQ, by Tim Mann email@example.com with numerous contributions by others.
a) First, be sure you have the latest versions of GNU Chess (and XBoard
or WinBoard, if you are using them); see topic [A.4] above. Several
different bugs that could cause this symptom existed in old versions but
have been fixed in the latest ones.
b) Another possibility is that you have a persistent transposition table
(hashfile) that has been corrupted. Look in the LIBDIR directory you
defined in the GNU Chess Makefile, and if you find a file named
gnuchess.hash there, remove it. Do not use the hashfile if you are
running multiple copies of GNU Chess at the same time (for instance,
with Two Machines mode in XBoard). In fact, it is probably best not to
use the hashfile under any circumstances.