it aint over till the fat lady sings

It aint over till the fat lady sings is a very common phrase which is used when you want to say that nothing is irreversible until the final act is played out. This is why it is related to a fat lady singing; a fat lady is a stereotypical term for an opera singer so in this phrase, the person is saying not to give up or quit until the last step of the process is complete because you never know what can happen until it is over.