After playing glhack, I noticed this in my cron output:

/etc/cron.monthly/debsums:
/var/games/glhack/logfile
/var/games/glhack/record

It looks like glhack overwrites files shipped in the package:

$ apt-file show glhack | grep /var
glhack: /var/games/glhack/logfile
glhack: /var/games/glhack/perm
glhack: /var/games/glhack/record

Instead, they should be removed from the package and created by the
game when it starts and removed by the package purge process.