I got randomly selected for the long one in 2010. There were several questions I was not able to answer but providing incorrect information is a crime and not answering is a crime. So I did nothing. They sent one or two threatening follow ups. Then an in-person enumerator showed up and ran me through the short-form questions only, which was fine.
https://www.politifact.com/factchecks/2014/jan/09/us-census-...