What will reverse polarity in an Energizer AAA, unpowered, dead battery, circuit?
If one battery has less capacity than the others then it will drop to zero faster and may reverse since the current through the load is going through the battery in reverse. 

In other words, it's possible that two good batteries reverse charged a weak one. 

This would be particularly likely if Mom replaced two of the three batteries and left an old one in there (perhaps because that was all that was at hand).