At 1:18 into the film, before the rats attack Mr. Martin, the plaque on his desk with the quote "Do Unto Others Before They Do Unto You" is between the Paymaster machine and the telephone. At 1:20 into it, as the rats attack him, the plaque is to the left of the Paymaster machine.
When Joan gives Willard Chloe the cat as a pet, she tells him that female cats are good company. Chloe was actually portrayed by a male cat.
When Mr. Martin honks the horn of his Lincoln at, and pulls up alongside, Willard, the car has no rear view mirror.
When Willard goes to the sink to add water to the rat poison, the rat wrangler's shadow can be seen as he reaches to remove Ben from the center island.
Early in the film, Willard plans on drowning the rats, apparently not knowing that rats are very good swimmers (in fact, the average rat can both swim half a mile and tread water for three days). They could have easily swam across the pool from the "island" and escaped.