It varies but there is nearly always some kind of fraud going on.  Sometimes there are goofy things that we will just have to forget about.  Other times it is a pretty good sized problem.  ACORN worked voter registration in New Mexico a couple of years ago and registered dogs and young teens.  The thing that surprises me is we do not take this precious right seriously enough to be sure these things cannot happen.  And when you suggest something should be done the response is always, you are trying to disenfranchise someone.