I would guess that in virtually every polling place in every state there were violations of election law and this is true of every election.

I worked as a poll inspector one presidential election year while I was in Law School. I was never trained for the job but I was given a certificate saying I had been trained. None of the workers had any clue about the election law or procedures. At one point I walked over to a machine and found one of the Democratic Poll inspectors in the booth with a voter, while the two republican inspectors (Two college girls who had no clue)(This was an area in which registered republicans were a huge minority and it they couldn't even fill all the positions) just watched.

Most violations are not malicious on the part of the poll staff, sometimes they don't understand, other times they are afraid to make waves.

I had two Democratic Poll inspectors insist that we count the absentee ballot of a person whose voter registration card wasn't signed (It had a photocopy of her signature cut out and taped to it). They weren't thinking about vote fraud, they were thinking that this was some young kid who had forgotten to sign the registration card, and her mother or father just put her signature on it to correct the mistake and why shouldn't her vote count.

The machine breaks and no one on sight knows how to fix it, so you take turns kicking it until it works again.

So when someone does commit fraud it can get right past the watchers because they aren't expecting it and they are not trained to stop it.