My favorite example of this is an exhibit that I saw at the Carnegie Museum of Natural History in Pittsburgh many years ago. There was a diorama of several forest animals, and an interface that shined lights on animals with different features. The "lays eggs" light shined on an assortment of animals including a Rabbit. Rabbits don't lay eggs, they only deliver them to good boys and girls.

We pointed this out to a worker that day. Several years later, we went back to see that the exhibit had not changed. I'm not sure if it's still there today.

Obviously, an easter egg. Well done Carnegie Museum! :-)