I had same happen to me a few weeks ago.
Charging a pair of fairly old, standard NiMh AA, energiser cells (silver wrapper with green writing), one charged fine, I caught the other a while later, with many more mAh recorded more than the first and/or the capacity of the cell (I can’t remember, but it would have been around double or more to shock me like it did)… Cell was extremely hot to touch and wrap was blistering.
Must say, as I’ve not had the UM2 long, I’m pretty annoyed… I don’t get why everyone is blaming the cells, I’ve charged these many times with Zanflare C4, Nitecore D2, and other multi-chemistry chargers without issue, yes I’m sure high internal resistance makes it difficult, but Nirecore consider themselves a premium brand, and should be capable of making a safe charger…
Do these things not have temp protection in them or what? Shame HJK rates it fairly well on his comparison site if it has such massive problems…
Edit to add: I didn’t actually see if the display had switched from NiMh to Li-ion, I pulled the cell and unplugged the charger extremely quickly as I was very concerned.