TL;DR During updates, Mongo moves a record from one position in the index to another position. It does this in-place without acquiring a lock. Thus during a read query, the index scan can miss the record being updated, even if the record matched the query before the update began.
Seriously. I was looking at DB usage statistics recently and was appalled MongoDB is still so popular. I thought it was done, nail in the coffin, when https://www.youtube.com/watch?v=b2F-DItXtZs came out 6 years ago, I haven't followed it much since then apart from the occasional post like this whose content is just "you thought it was bad already? haha it's worse."
