Say I'm making a thing which uses some GPIOs to launch a rocket. I wouldn't want to launch the rocket because there was a power outage and the Pi rebooted. As such, it's important to know the startup behavior of the GPIOs.
As an example of some undesirable behavior, I could trigger a launch when GPIO 14 goes high. GPIO 14 is perfectly usable, and can be controlled like any other pin. However, it goes high just for a bit when the Pi is booting. This would inadvertently launch the rocket whenever the Pi reboots.
So, which GPIOs have startup behavior I can rely on, and what is that behavior?
I should note this seems to be a common question, so I expect people will want to close it. So allow me to preemptively explain why the existing answers are insufficient.
A common answer is "GPIOs 0-8 pull high, and GPIOs 9-27 pull low.". This seems to come from the BCM2835 datasheet and is reinforced by the HAT design guide. This may be a correct answer for a BCM2835, but a Raspberry Pi is more than that. It may even be true for a Raspberry Pi with no software, however I think most people will understand "Raspberry Pi" to include an installation of Raspberry Pi OS, which if I had to guess, is what's responsible for the activity on GPIO 14.