I was trying to work out what was wrong with a device which wasn't functioning. The first thing I checked was the batteries. When I tested the first battery with the multi-meter it was reading a negative value. I checked my multi-meter leads were connected correctly and tested the battery again. It still read a negative value so I tested the other battery and it was reading the same value only it was positive. These were standard non rechargeable alkaline batteries. I didn't know it was possible to reverse the polarity of a battery let alone a non rechargeable battery so I did some research to find out how its polarity reversed.
The device took 2 batteries and they were connected in series. Additionally as the device had been playing up even though it was not running it had been left with the power switch in the on position. It turns out that if batteries are connected in series and one discharges before the others then a reverse charging current will flow through the discharged battery. This is bad because it can cause the gas pressure inside the battery to rise leading to leakage, deterioration in performance, swelling or rupture. It can also result in the weaker battery becoming reverse charged.
This is why its important not to mix new batteries with old batteries, rechargeable batteries with non rechargeable or even batteries of different brands/model batteries as they could discharge at different rates or have different voltages to begin with.
Reference: Physics Forum