Elastic giving way in public: a common occurrence in the past
Until man-made fibres came in sometime during the 1950s, it was quite normal for underwear such as knickers and half-slips to be held up with elastic which was threaded through the hem of a waistband and secured together at its ends, either by sewing or knotting. The elastic was not sewn onto the fabric itself. In use, it therefore came unsewn or undone after a time, so that there was nothing to hold up the knickers of half-slips.
If a girl or woman was sitting down at the time, and felt the elastic 'go', the knickers or half-slip could be hitched up for a time by holding through the top clothes. If not, the item of underwear simply dropped around the ankles in full view of everyone. Every girl and woman at the time experienced this at some stage and every boy and man at the time must have seen this happen, even though the polite thing to do was to avert the eyes.
When it happened, there was nothing for the girl or woman to do but to stoop, step out of the item and pick it up. I am told that this even happened to my school headmistress during school assembly - a story never to be forgotten by the girls who saw it, although it was before my time.
Of course, this use of elastic meant that new elastic could easily be threaded through, so that the item readily became useable again. It was a 'make do and mend' society. With more modern, 'sewn in' elastic, items have to be thrown away when the elastic loses its strength.