http://wiki.squid-cache.org/SquidFaq/TroubleShooting
Referencing to Tuning Kernel Limits of the FreeBSD based on Adrian Chad article.
The docs describes that the basic "server accept" socket is bounded to a queue of 128 connections.
You would probably see something like "connection reset by peer" and you will need to increase the kern.ipc.somaxconn to 2048 to match something useful for production network of about 300 users.
In a case you have a loaded server you would need to increase it past the 16384 limit.