let's say the setup and hold times are both 1 time unit
if you meet these conditions, everything works as expected according to the various data sheets for the various parts
if you don't meet one or both, you run into the various race condition possibilities and what happens
is at least a temporary instability in some gate
at worst? maybe a latch up and blue screen of death,
or (for example) a power converter could (conceivably) turn on all of the power switches at the same time
and burn itself up