Im sure it probably has happened where a climber went down the wrong side, though I dont know of any specific stories......
Trunl (IM BACK!!!!)
Don't be sure as it has not yet happened. You not only need extra permits for both side, but you will also be jailed for a long time if you cross the border illegaly.
This year some climbers from Nepal finally got all the necessary permits, but the weather was too bad for them to climb...