Sorry, but I cringe every time I see someone suggest this.

Startup scripts often aren’t idempotent, and it takes work to make sure they are. Please don’t source your startup script unless you know what you’re doing.

Any examples of non idempotent script someone is likely to write? Because I can't think of much apart from doing some logging like 'user x logged at time y' which is likely to be fine to be written twice because it would be anyways if you open new terminal.

Yeah i'd like to know some examples as well, I've never had a problem with this