Neither quelonzia nor I remember filling out a census form in 2000 -- and in 2000, we were living in CSUMB's student housing. Despite the fact that we lived there for five full years (and the longest I've ever lived in any single place is six years), we were considered transient. In many places, students living on campus aren't considered "local residents", and thus aren't permitted to vote in local elections (though they can vote in national elections); since districting is based on census data, it makes sense that they might leave students out of that, as well.
In 1990, I was in the Coast Guard, and lived on a military base.
In 1980, my family lived in an RV park in a largely-agricultural part of Southern California; the immigration status of most of the other long-term park residents was, shall we say, dubious. My mother can't remember if we filled out a census form that year.
This may be the first census that's actually counted me since I was six years old.
The next time someone bitches about the effort the government has been going through to try and get the homeless and other "traditionally under-represented" segments of the population tallied accurately, I'll have to point out how easily a middle class white kid slipped through the cracks for forty years.
(This may be the most appropriate use of my barcode icon ever.)