Someone wrote this comment: "I think the biggest peeve of mine here is that people have no sense of others around them." I replied with the following post:Later, I realised that I had changed from the plural People understand ... they are taking ... their to the singular rucksack is. Why did I do that?People understand intuitively how much space they are taking up - but not how much space their rucksack is taking up.
I consulted a book I often use, A Dictionary of Modern American Usage by Bryan A. Garner. He says that this "seeming sloppiness" is particularly common in British English (my English) and is afraid it is going to invade American English. He quotes several examples; this one is with his corrections: "In school, seats are not assigned, yet students tend to sit in the same seats or nearly the same each time, and sometimes feel vaguely resentful if someone else gets [read others get] there first and takes 'their' seat [read take 'their' seats]."
In the last few days I have heard this pattern twice. I heard myself say to Mrs R.O. that we shouldn't do something "in case we change our mind." I heard someone on a radio programme say, "Many new mothers are frightened they'll harm their baby."
In my original sentence, about people with rucksacks, I think that while writing my mental image changed from people in general to a single person with a rucksack; so the singular was emotionally right but grammatically wrong. Perhaps this explains the other examples.