Did the last name get dropped?

Does your ancestor have a middle name that is also a last name? Is it possible that he cannot be located in a census or other record because the census taker left off his actual last name and used his middle name as a last name?

It might explain when Henry Jacobs Fecht is enumerated in the 1870 census as Henry Jacobs.

And if you don’t know that middle name it’s going to be even more difficult to find him.