In the tradition of fine bureaucrats everywhere, someone determined that census workers must have background checks. Apparently the procedures don’t tell them what to do when someone fails that check, so at least in one case, they just ignored it.
A woman in NJ recognized her local census worker, going door to door through family-friendly neighborhoods - because he was a registered sex offender. After the cops arrested him, it was determined that he had failed the background check, but he was trained and sent out anyway.
On a cheerier note, here are a couple different ways to deal with census workers when they knock on your door.