As some colleagues of mine found out this is not the case. On the ground they entered the DOW instead of the ZFW which gave a weight of around 13T less in the FMC. When trying to climb to FL380 whilst showing MAX ALT of FL385 i believe ,the a/c reverted to LVCH and started to descend.After investigation they realised the wrong entry. So i beleive its the good old saying garbage in garbage out!!