Answer: A cracked bottle.
99% of the time a water cooler leak in due to a crack in the bottle. A tiny crack or hole in the water bottle can be caused by rough handling or dropping the water bottle. The cracks can be tiny, sometimes even too small for water to escape from, but large enough for air to enter under suction pressure. The air coming in will allow the water to fall and overfill the tank, resulting in a leak. If this occurs the water must be drained from the tank before the replacement bottle is put on, or this will spill the excess water out and give the impression that the leak is still happening. Mark the cracked bottle as faulty and we will replace it free of charge in the next delivery.