I have an Eheim 2126 Pro II for my Malawi Cichlid tank for the last 3.5 yrs. Recently, it leaked while I was doing some electrical work in the house when I turned the power off. I have hardwood floor and it must have spilled a pint of water before I realized that it was leaking.
Apparently what had happened was the o-ring seal on the canister wasn't holding the water in when power was cut. The filter was in a cabinet underneath the tank, and when the filter isn't running, the water pressure from the inlet and outlet hoses would create enough force to leak water out of the canister. This wouldn't happen when the filter is running normally (since the pressure is balanced), but once you cut power, the pressure is no longer balanced.
Lesson learned: keep the seal in good shape, and put the canister filter inside of a waterproof bin as a backup! Nevertheless the Eheim is a fine filter, absolutely quiet, and works well otherwise.