The bag of rats can be used to exploit several loopholes.
The original is a fighter carrying around a BoR with the purpose of throwing them at an enemy, then using Whirlwind Attack to attack them all in one round, and Great Cleave to get a free hit at the actual enemy for each rat killed.
This problem was address in on page 40 of Fourth Edition's Dungeon Master's Guide, addressing legitimate targets, specifically using the bag of rats as an example. Stating "For instance, characters can gain no benefit from carrying a sack of rats in the hope of healing their allies by hitting the rats."