Examples:

• I’m playing Go Fish with my niece and so far she’s winning, but I haven’t given up yet! It ain’t over till the fat lady sings.
• It's not looking likely that the senator will be elected to another term, but it ain't over until the fat lady sings.
• Who knows, Michael might fall off and break a leg,' he joked. `As they say, it isn't over until the fat lady sings.