Parents can jigger things up by running late as well......
I had one Parents' Evening where I was set up for 10-minute slots, with a break mid-evening at 7.00pm. In theory, we could run up until 8:30 - 9:00, but bearing in mind that I usually arrived at 7:45 am, and taught all day, I'm not sure how coherent I was by 9:00 pm.......
When the requests came in, by some fluke, everyone got the spot they asked for in the first half of the evening; one parent asked for another day because he was a deputy head with his own Parents' Evening on the same day! Fair enough. One pregnant lady was highly imminent, so I saw her at morning drop-off a few days earlier, and as chance would have it, she had twins, so two interviews for the price of one, two others came in straight after school on other days.
On the evening in question, I had the schedule posted outside the door, and while they waited, folders were laid out on the chairs outside with the latest test papers and a few other samples, so they were gainfully occupied while waiting, I had thumbnail sketches to hand from colleagues who taught their sons in other subjects.
One couple had the 5:40 slot, but did not arrive. No worries, but no message to advise of traffic jams or the like, so I moved on the the next on the list. I worked my way steadily through the list, calling for them again each time I called in the next parent, but no sign, and no message to say that they were on their way; I would have waited if there was a problem I knew about.
My break time was over, and I had accounted for all the other parents, so I explained the situation to the head teacher. He agreed that I could reasonably call it a day; quite honestly, after a certain point, you have trouble remembering who you are, let alone anybody else.
Apparently, they turned up 10 minutes later, and were gobsmacked to discover that I had gone home. I think they were working on the assumption that the appointment times was a mere formality, and I was basically at their disposal at any time during the 5-hour time period which suited them.