Mothers who are sick sometimes display wrongly, as well. Usually it means their babies just disappear, to appear in their arms again magically once they are healed. Sometimes, however, people have reported the wrong object being displayed, like a fish. People here have posted a couple of screenshots of such instances in the past.

It doesn't seem to be a very common glitch, but I remember spotting a villager by the hospital and I saw that she was sick, so I brought over a doctor to heal her. I then dragged her to the field as she was a farmer. She just stood there dumbly and a baby suddenly appeared; her details showed that she had only about half an hour to go. The same sort of thing happening on start-up is probably slightly more common, but still not frequent, certainly not worth worrying about.