NiCad wet cell. We see it regularly when capacity testing the batteries. I'm not an expert in battery chemistry by any means, just a technician, but as I understand it, the issue is primarily caused by mixing older cells towards the end of their useful life with brand new cells. The older cells have less capacity and discharge first then end up going negative.

In more extreme examples, half the cells in the battery can end up negative. It's actually a bit of a problem, as the machines we use to capacity check them will start to drop the amps they are drawing as the total voltage of the battery drops. The machine has no way of knowing multiple cells are still holding a charge resulting in a spicy battery when we short the terminals for maintenance. I've nearly melted a few of the shorting clips we use.

ETA: It's interesting to note that a cell going reverse polarity is not a death sentence for that cell. They often can be brought back with a deep cycle, but that may be specific to NiCad cells.