I've heard many misconceptions in my time as a zoo-goer, but most of the comments were more uneducated than directly stupid (like calling an african wild dog a hyena etc. (by the way, an african wild dog is called "hyænehund" in my native language which means "hyena dog")
The worst I've heard, however, was this one: In the small Danish zoo Jutland's Park Zoo, an elder woman was walking with two small kids (I think she was their grandmother). I was watching the ring-tailed lemurs. This day, they were unusually active, jumping around, play-fighting, playing etc. Soon, the woman and the kids arrived. The kids who couldn't read pointed to the sign where they clearly was written "Ring-tailed lemur" (Kattalemur in Danish) and asked "What is that animal?" The woman didn't even bother to read the sign even though her grandchildren was pointing at it, and just stated "seems like a sloth".
What's worse, the irony in this is that a sloth in Danish is "dovendyr" which literally means "lazy animal", and the ring-tailed lemurs were probably the most active animals that day.
Near the lemur island is a paddock with alpacas and cameroon sheep. Though not as embarassingly ignorant (pardon my honesty) as the comment with the lemurs, I heard the woman refer to them as "dromedaries and goats".
On a positive note about these kinds of people, I like going to the zoo with my dad. He's not knowledgeable about animals and he tends to the forget the names of the lesser well-known animals he has seen in a zoo. But when he is in a zoo, he can be hard to pull away from an exhibit because he spends long time reading every single sign in order to find out what he's actually watching. This means I don't have to be an annoying know-it-all, only that I maybe can add "this animal is actually very rare in zoos" etc.
So all in all, I don't really care if people don't know a jolt of what they're watching. But it can bother me if they're too ignorant to want to know it - why then go to the zoo?