That's not true, thebody. If, as we believe, Heswall is a surgeon/anaesthetist or operating theatre nurse, she cannot leave in the middle of an operation. It is very rare for a surgeon or scrub nurse to be relieved during an operation, for reasons of safety and continuity of care, so if a theatre list runs late - because an earlier case has complications that make it longer than its predicted length, or because an emergency case is put into the list, or because one of the surgeons is called away to deal with something else - then the list can, and often does, go on past the time it's supposed to finish - and basically the staff in that theatre have to stick with it until it is done. The surgeon then has to write up operation notes and may well have to see other patients, and the nurses have to clean up the theatre and see to the instruments. If you are lucky, someone will come in to help you or relieve you, but in my experience, you basically just had to finish on your own.
And this is by no means the only job that has unpredicatable elements to it, and many people have jobs that they cannot simply walk away from at 5pm. If the phone rings with something urgent, or a minor crisis occurs, it may mean that a parent has to stay to deal with it.
However, no-one has the right to be late without explanation or apology, or on a regular basis without prior arrangement; and communication is vital. As Happy said - had she known earlier that her employers were going to be late, she could have made alternative arrangements and not missed her son's birthday party. And if you have the sort of job where you may be detained at work past your nominal finishing time, then you make this clear to any nanny you employ, you find one who is happy to be flexible, and you remunerate her suitably for that flexibility - and crucially you don't take advantage of her flexibility - so if she has another commitment/appointment/family occasion to go to, and does need you to be home on time, you make sure that happens.