My bike light uses two CR2032 batteries, which I replaced with CR2025 and some padding. It stopped giving light, so I removed the batteries and measured voltage. One battery gave 2.8 V, and another -1 V. I checked with new batteries that the polarity was right and confirm that the battery had negative voltage. I replaced with a new CR2025 and the previous positive battery and the light worked again.
The light was working before, so the "negative" battery had to be around 1.5V, and I did not notice it being backward inside the light when I removed it.
How can a battery reverse its voltage like this?