I'm not a battery fundi, but I know that many dry and rechargeable batteries have this tendency when going completely flat. It is a chemical reaction that goes awry on the inside. If it's an ordinary dry cell, chuck it away. If it's a rechargeable, maybe it can be rejuvenated!

Multi-cell batteries (batteries that have multiple battery cells in series) can run flat at different rates. In your case if several batteries are connected to their load in series (like a long flashlight) it is possible for one of the batteries to go bad and reach zero volts before the others are discharged.

When that happens, the current that flows through the dead battery is actually in the reverse direction compared to when it was charged. The voltage potential across the dead cell is now backward. This reverse current places a reverse charge on the dead cell and the dead cell's voltage charge becomes reversed.

This is bad for a battery and they can get hot and actually blow up in extreme cases.

Thanks to both of you for the explanation! Ried

Exactly it happens in cheap batteries who's cells are not separately group-tested for their for capacity & internal resistance.

When reverse charged it opposes the remaining cells in the group. So you minus the voltage of bad cell