It's not over till/until the fat lady sings American proverb
The expression is used in sports contests.
The last result of a given situation, event, etc. can be changed until it is completely finished.
Team B is leading by two sets, but we'll never give up. It ain’t over till the fat lady sings.
Some people think our team will lose in this competition, but it ain’t over till the fat lady sings.
The phrase has a connection to Opera. A source recorded that it came from Ralph Carpenter in the Dallas Morning News in 1976.
A legal matter that is very easy to solve
The house owners didn't lock the doors before going out, and that's why they got broken in. This is an open and shut case.