While you'd never mistake a housecat for a full boar, mistaking a dead cat in a sack for a dead piglet wouldn't be that difficult. Especially if you'd been showed the pig before and the cat was only substituted in at the last minute to provide weight and legs.

Additionally, I'd point to the "pig in a poke wiki" page.

http://en.wikipedia.org/wiki/Pig-in-a-poke

The page shows that variants of this idiom are present in hundreds of languages, often with the statement that a person bought a cat instead of a pig or, occasionally, as rabbit. In fact, more culture refer to buying a cat in a sack than a pig.

The wiki page mentions your cat in the bag theory as well. However, if you follow the references, you'll considered that it is seen as dubious and that the pig in a poke explanation is considered far more likely.

It's a little more complicated than that. The pig, in this case, is usually a piglet, which is roughly the same size and shape as a cat, and the trick is to show the customer/mark a real piglet being put in a bag, and then use sleight of hand to switch bags and hand him one with the cat in it, with the strong admonition not to open the bag before getting home lest the "pig" get out.