Occasionally the X server on our LTSP terminals spontaneously switches keyboard
layout from  gb(pc105) to American so that, for example, the hash key (#) comes
out as backslash.

One one occasion when this had happened I discovered the following message in
Xorg.0.0.log:
  (EE) Couldn't load XKB keymap, falling back to pre-XKB keymap

I have also noticed many files in /tmp/samba with names like ip.address.log.

My analysis of the problem (partly based on guesswork) is as follows:

(1) whenever the X server is reset it recompiles the keymap files into the
directory /usr/X11R6/lib/X11/xkb/compiled

(2) Because of links this directory is actually in the /tmp filesystem, so if
there is insufficient space in /tmp the keymap file cannot be recompiled.

(3) Whenever an SMB client attempts to connect then a log file is created in
/tmp/samba. This can happen a lot if you have a large network with many Windows
boxes, especially if their personal hygiene is imperfect.

My solution is to reconfigure Samba not to create unlimited log files, and I
will attach a patch to /etc/rc.localdev which does that.