If you have a thermostatic mixing valve installed on your shower then the water and shower may turn cold or nothing will happen if you flush the toilet while someone is in the shower.
But you may experience a drop in water pressure in the shower if the toilet is flushed.
Now if you don't have a thermostatic mixing valve in the shower and you flush the toilet then the shower will lose some cold water pressure which could cause the water to get hotter in the shower and potentially burn the person taking a shower so it's not a good idea to flush the toilet while someone is taking a shower although it is funny.
But it won't be funny if someone becomes burnt because of it.