1. Mixed use: when batteries are used from different manufacturers, different type i.e. (zinc-carbon together with alkaline or others), different grade or a mix between old and new, there is a high risk of over discharging one of the batteries. Due a difference in capacity it is possible that with a mix of batteries as above stated can cause one of the batteries to over discharge, with high risk of leakage of that over discharged battery.
2. Leaving battery in appliance: when batteries are left in an equipment which is not turned off, or left for longer period in an unused appliance, there is a chance that these batteries will run empty and if kept too long in the appliance they might over discharge which has a possibility of leakage as consequence.