A “nit” is a louse egg. On its own, it is nearly invisible and unlikely to trouble you. But left alone, it hatches into a louse. You can probably survive a louse or two quite easily. But if a couple of people in a community each have a louse or two, they breed. And eventually, your community is full of lice, and then it is very difficult to get rid of them. Monkeys groom each other regularly, picking at the nits, thus keeping them below the level where they can bloom into a full-on louse infestation.
The metaphor applies to coding style, edge cases, questionable security practices, untested code, and grammar.
picks_at_nits on Hacker News