At squid with Debian Lenny you see in /var/log/squid/cache.log something like this:
2011/05/21 07:55:59| WARNING: Disk space over limit: 105172 KB > 102400 KB
If you check the diskspace you see that there ist enough space to save files.
Possible causes:
crash of the system, power failure, error in the program
Stop Squid
# /etc/init.d/squid stop
Delete file /var/spool/swap.state.
# cd /var/cache/squid
# rm swap.state
Start Squid
# /etc/init.d/squid start
Source: http://www.squid-cache.org/mail-archive/squid-users/200306/0334.html