I had the same problem, not with the 1930 census, but 1870 and 1900. In the 1900 incident, I found my maternal 2nd ggrandfather's family in the exact place they should have been, but they had the wrong surname. I can only guess that perhaps they had listed them under his wife's surname (since I have never discovered her maiden name)for some reason.
In the 1870 incident, I realized that apparently the census takers missed an entire neighborhood. I had several sets of ancestors all living in the same area during that time, but in 1870, I only found part of one of those families which should have included all of the individuals because they all lived right beside one another thru every census I had ever seen from 1800 right on thru the 1900's, including to this day even. After researching other families in the neighborhood, I noticed that they were also missing, several pages worth of families were just not there, or anywhere else in that 1870 census. That's when I realized that the area had apparently been skipped over that year. I do know that they were fairly close to the county & different township boundaries (they'd been listed back and forth as different townships in some of the years, but they had not really moved) and suspect there was a mix-up about boundaries that year and perhaps the entire neighborhood was just overlooked.
So, who knows, maybe there were other places that faced similiar circumstances.