This seems like a no brainer question but how detrimental is putting cold fluid into a hot engine? I was reading a thread on here about changing coolant and someone said to ensure the engine is cold when doing so which makes sense but I remember when I was an apprentice at ford alot of the other mechanics (even the forman) wouldn't allow much time for the engine to cool before dumping the coolant and filling it back up.... I fondly remember this when the forman changed the coolant on an AUII falcon wagon and when the 2nd year apprentice replaced the thermostat on an SS commodore...

I'm sure by the time they put new coolant and water in the engines they would have dropped below operating temperature but would still have been relatively hot... the cars seemed fine and drove normally after having this done but it it possible for them to have done damage without having any immediate signs of anything wrong?

the worst thing about coolant and a hot engine is opening the cap with hot water under pressure and getting burnt, and also if the cars out of water and its extremly hot and you fill it up, the water instantly turns to steam and can burn very badly, also you can crack the head from putting cold water into a very hot engine, thats about as much as i know

it can warp or crack the head..
in times of need you can add cold water/coolant but the engine must be running and the water must be added slowly...

as for mechanics ect doing it, more than likely the engines are only warm not hot..
no mechanics like working with hot engines, especially hot coolant..

Pretty much this, you can never work on a hot motor comfortably or safely, they would be below the optimum temperature for sure.