To elaborate more:
I remember that when starting their commando raids the British encountered some troubles with the rules against restraining POWs. For the Dieppe raid they had issued handcuffs and orders to restrain POWs, who might try to run away basicly minutes after initially surrendering or dispose interesting pieces of equipment the military Intel guys are very keen on getting their hands on, until being removed from the combat zone and thoroughly searched. When that raid failed the Germans found these. (Written Orders and Handcuffs)
To protest against this clear war-crime the Germans started handcuffing some British POWs in one of their camps (another obvious war-crime) and letting the Brits know what was going on. The Brits then started to handcuff a identical number of German POWs in one of their camps. (Yeah yet another clear war-crime)
That affair lasted a few weeks until both sides agreed that restraining POWs within a combat zone may not be okay by the letters of the law but is far better than doing it by the letters of the law and shoot some of em in that process. (Shooting POWs legally because these misbehave)
Submarine warfare was something similar. The way submarines (on all sides) operated in WWII was in clear violation of the Haag-Convention. But everybody realised that these rules where obsolete by technological advances and was OK with a modus operandi that has to be considered a war-crime but was a quiet ethical approach in adapting the aim of that law to the conditions in the battlefield with all the technological advances. In the Nürnberg-Trials after WWII grand admiral Dönitz was charged with that violations but not found guilty (in spite of having committed these crimes which was easy to proof) on grounds of ‘yeah he did but so did we… we let it slip’ arguments.
Where there other / more such situations where the warring nations stumbled over parts of the laws to keep war human and had to adapt?