It can happen to us all...
I regularly fly to Durban (from Johannesburg) & return the same day. About 6 months ago I went through security at JNB boarded and flew to DUR. Spent a long day working and rushed back to the airport to catch my homeward flight. I cut it quite fine (pun intended!) as I had my boarding pass and had no luggage to check in. Got to security took my laptop out its bag, walked through, was frisked down, got my laptop, but the bag had gone absent. Then I was asked if the bag was mine and informed that there was a leatherman in it. Sure enough I found my Leatherma wave in the side pocket. There was no time to check it in (not that it would have got to JNB anyway) so I had to put it in the large disposal bin!!
No system is 100% perfect, but we should be able to place a high level of reliance on it.