Re: metastability
when all the I/P constraints for a flop such as setup & hold time are meet, then FF produces faithful O/P. However when any one of these constraints is not meet then O/P gets into metastable state which we also call as unstable state (undefined state) from there it may achieve logic O/P as 1/0 & we normally say that O/P state can not be predicted but high end designs can predict the O/P state after it once achieves metastable state I've some stuff about this I'll attach that shortly