When inserting the batteries in to the battery case carefully follow the diagram on the battery case or in the instructions in regards to the batteries polarity.
A battery inserted the wrong way can explode.
If the polarity of one battery is reversed in the battery case (with two (+) ends in contact with one another or two (-) ends in contact with each other) it's will be charged by the other batteries.
This causes a chemical reaction inside the reversed battery. Within a few minutes, the battery can leak explosive gases and/or extremely corrosive liquids.
There is a danger of explosion and chemical burns.
Indication: a weak level of light
If a torch with fresh batteries emits a weak light, it could mean that one or more of the batteries are reversed.
What to do?
Immediately turn off the headlamp and check the polarity of the batteries.
Precautions must be taken when opening the battery case in the event a leak has already occurred or occurs while opening the case. Wear protective gloves and goggles. If you are unsure do not open the case, especially if you are in a remote location. Situations like this are a good reason for carrying a spare torch, like the Petzl e+lite.
If you come in contact with the liquid from the batteries, immediately rinse the affected area with running water and seek medical help.