If you use 2018 unity version or higher and when you fire a weapon, it shows an error message, it is due to the new collision detection on unity 2018 and higher.

To fix this, search the projectile prefabs in the project and configure the collision detection to deterministic in those projectiles which uses a different collision detection from discrete.

If the collision detection is discrete, there is no need to change the setting.

This is a bug from unity itself, in other versions like 2017, this won't happen.