It ain't over till the fat lady sings
It ain't over till (or until) the fat lady sings is a colloquialism which is often used as a proverb. It means that one should not presume to know the outcome of an event which is still in progress. More specifically, the phrase is used when a situation is (or appears to be) nearing its conclusion. It cautions against assuming that the current state of an event is irreversible and clearly determines how or when the event will end.