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 (see also "Why do I need to use the same type and brand of battery?").
Leaving battery in appliance: when batteries are left in a device which is not turned off or left for a longer period in an unused appliance, there is a chance that these batteries will run empty. If kept in the appliance for too long, they might over-discharge which has a possibility of leakage as consequence.