I volunteered as an Assistant Clerk in my county for the last primary and presidential election.  I can confirm that the system we used allowed for potential voter fraud, although I cannot say that I saw any evidence of it myself.  When we challenged our trainer from the Supervisor of Elections on the obvious potential for massive fraud, we were told that if they discovered that someone committed voter fraud they would be prosecuted, and we were not to concern ourselves about this.   Most of us were left dumbfounded by the lack of concern of the Supervisor of Elections.