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.
You might be thinking of 2000, there was no long form in 2010. Could you not answer because you didn't know, or you didn't want to answer those questions?
> Could you not answer because you didn't know, or you didn't want to answer those questions?
Didn't know many. Didn't see how the Census clause authorized the collection of personal and private information they were demanding I turn over accurately under criminal penalty otherwise. Not knowing was sufficient to not be able to respond.