We are terrible at remembering faces shown to us fleetingly. This video, for example, admirably demonstrates this effect: 4/5 people pick the wrong guy!
On a trip from Denver, we were unfortunate enough to see this play out in practice. We were returning from a very satisfying trip to Yellowstone National Park. After returning our rental van (4 adults and 2 kids), we boarded an Avis bus that was supposed to take us to the terminal. We were tired, anxious to get back home, and somewhat distracted.
There is always plenty of distraction when travelling with two little kids.
There were three other people in the same Avis bus: two guys and a lady. One of the guys (who turned out to be a thief) even dropped something during the trip. I watched him get out of his seat, pick it up, and get back in.
After the bus stopped, one of the guys and the lady got off, and I almost bumped into the thief, and politely gave him the right of way. He said, "after you, please", and I got off, and ran to a get a trolley to haul our luggage.
When I returned, I saw that one of our backpacks was missing. I tried running into the airport terminal to find this guy, but I'm embarrassed to confess that seriously I only had the vaguest idea what he looked like.
And I had two "good" looks at him.
This memory effect is especially not good when you're in a sea of people at Denver International Airport.
In any case, we lost a couple of passports, which led to some irritation, and an old digital camera.
More significantly, I lost a little more trust in strangers, which unfortunately, is less easy to replace.