For seven years, I was RD for a half marathon. People would often finish with no bib. Usually, it was a first timer. The reply: "I didn't know what it was for. It's in the car." I let them get the number, show me, and that was that.
We used to have tags on the bib which we pulled off as they came through the chute. Beginners would have just the tag pinned on. Again, "I thought this was all you needed. " And conversely, the beginner who had removed the finishing tag, saying "I thought that was a receipt." We kept 3x5 cards to mark the finish for these people.
Then, there's the beginners who pinned the number to their back, when told over and over it needs to be on the front so we can see it at check points.
And the bib on the shorts or shirt and covered by a second shirt, so we can't see anything. "Where's your number?" "Huh? What number?" Starts looking around at their clothes, finally pulling up the large shirt to reveal their number.
The worst problem: a finisher goes back for a friend and then runs through the finish together, getting a second finishing time, and throwing the results off sync.
It was a small race. The average was 75 finishers. Our big year had 91.