We have seen this happen for two reasons:
1. Disk full. Remember, if the squid is running with non-root privileges (strongly recommended!) it will only be permitted to fill a partition to 95% capacity. Adjust the limits in squid.conf accordingly, then restart squid.
2. Network restart. If a request comes in while the network is unavailable, squid will make a few attempts and then proceed with a clean shutdown to prevent the error logs from filling up. Simply restart squid to remedy the situation.
