This can happen if NanoFoil® activation is attempted and failed. I have seen this happen in a few instances when I used a dying 9 volt battery which had been strained from many tradeshow NanoFoil® demonstrations. I noticed that, with a new battery, the reaction was more easily started on a section of the foil on which activation had not yet been attempted. There is a threshold of energy needed to activate a reactive foil, slowly heating the foil will only allow the Ni and Al layers to diffuse - and raise the required activation energy.
In the picture shown here, notice the distinction between the light and dark layers. Each stack of one aluminum layer and one nickel layer is refered to as a 'bi-layer'. The bi-layer thickness and integrity are both important for the foil to activate at the proper temperature and release the proper amount of heat.
To learn more about using NanoFoil®, please check out the NanoBond® Process Series.