What could cause Ni-MH rechargeable batteries to leak?

It is generally said that Ni-MH rechargeable batteries hardly leak, unlike alkaline batteries.

However, both alkaline and rechargeable batteries contain electrolyte, which could induce leakage when the batteries are used incorrectly.

There are certain incorrect uses which could cause a sudden rise in battery temperature that generates gas which cannot be absorbed within the battery case:

• Forcefully discharging the batteries when they’re already over-discharged
• Really high current discharging can cause a lot of heat and eventually leakage
• Short circuit discharging; the battery will become really hot and will eventually start leaking

Safety valve: in order to prevent the batteries from exploding, a safety valve is fitted in every battery to discharge the gas. When this gas is being discharged there is a possibility that the electrolyte will leak out.