The U-120 story is an urban legend.Hard to believe the commander didn't know how to flush the toilet.

Sunk by a toilet?
In many sources it is stated the U-120 was "sunk by a toilet" (probably a very bad way to go! :). However this story should be attributed, with changes, to the U-1206 which was one of the late war boats fitted with the new deep water high-pressure toilets (enabling the boat to use its toilet at greater depth than before).

On 14 April 1945, only 8-10 miles off the British coast line, the boat was safely cruising at 200 feet when the commander, Schlitt, decided to use the toilet without the help of a trained specialist (the system was complicated). Something went wrong and when the specialist arrived he misunderstood something and opened the wrong valve with the end results that large amount of seawater got into the boat. The seawater reached the batteries directly under the toilet causing chlorine gas to form and the boat had to be surfaced immediately right under the enemy. When the boat reached the surface they managed to blow clean air into the boat but at the same time an aircraft bombed the boat causing extensive damages leaving the boat unable to dive. Seeing the hopeless situation Schlitt had no choice but to destroy his secret material and abandon ship to safe his crew. (Brennecke, J. (2001). Jager and Gejagte)