Depends on the car. There’s an infamous bolt on the transmission of 6 speed 2nd gen Miata’s. If you remove that bolt accidentally instead of the fill plug near it when replacing your transmission fluid, you will need a transmission rebuild/replacement. Likewise I would imagine if you shear off the crank bolt on a car while removing the crank pulley to do replace a water pump or a seal, you’re probably in for a heck of a tear down, and depending on how bad it is to get it removed, maybe a bottom end rebuild to replace damaged parts.

Heck we still live in a world where some cars with interference engines use timing belts instead of chains. Repair that 20 belt wrong and you will indeed need a whole engine, and that assumes your got to it before it broke and ruined your engine for you.