I didn't know this happened! Flicking through the book 'U-Boat Commander' by Peter 'Ali' Cremer I came across this, which I though I'd share as I didn't know anything about it:
The British Naval Intelligence, of course, knew the problems of long periods in dock from personal experience, for English submarine crews also enjoyed their home leave. The enemy (the British) used this situation for propaganda purposes, skilfully combining it with a call to our U-Boat crews to sabotage their own craft.
Out of a clear sky, small leaflets o bigger than the palm of a man's hand appeared in the bases along the Atlantic coast and in the new U-Boat base at Bergen. They lay on the ground in the bunker, were found in tool boxes or the canteen table. One fo these leaflets, printed in German and produced apparently in England read:
"SIX WEEKS LEAVE for the whole U-Boat crew
If a comrade at the base causes a ballast tank to split through excess air pressure
It's a sure and simple matter:
If a comrade closes the air vent and the flooding flap of a ballast tank, then slightly turns the corresponding air valve, as though to test whether the pressure delivery pipe is airtight, he can simply go away and wait for the accident' to occur.
It is best done during the stationary test at the pier or on trimming trial.
First, it has happened often enough without any assistance.
Secondly, no one can prove who has done it. Any stoker or dockyard worker coming aboard can have turned the air valve.
Thirdly, quite a time passes after the opening of the valve before the pressure becomes too great and the ballast tank burst. No one can say precisely when it happened.
One thing is certain: the boat will have to go into dock and the crew has increased time ashore!"