If the message was so important, why rely on only two soldiers to deliver it? It would have been more plausible if they had at least tried another method as well e.g. sending a delicious plump breasted pigeon.
As they were on such an urgent mission to save his brother, why was he focused on looking for food, especially as it later turned out that the other (at least) had plenty in his pack?
Would they really have risked their lives to pull a German pilot from his burning plane? And then after they have just saved said injured pilot from burning to death and when they are in the process of fetching him water, why would he try to kill them?
Why would someone with combat experience make someone stand up when they have just been stabbed in the stomach and try to drag them to an aid station when you don't know where it is and don't have the time? Saying "You can't go on, I'll take the message and send help/come back for you" would have made way more sense.
The Germans seemed to be remarkably poor shots. The sniping scene relied on the German having slow reactions and would have made more sense if our hero had changed position and not stayed on the steps and achieved a magic shot. He could have just moved along the canal and sneaked up on the house without first managing to hit the German when he was at a disadvantage and pinned down on the steps.
Why would an experienced soldier casually wander up to enemy troops (and run away from them) when he had a perfectly good rifle in his hands?
The scene where he ends up strangling the German - I absolutely expected that to be solved at the start with a rifle butt to the jaw, but perhaps army training back then was to put down your rifle and plead and wrestle with the enemy?
If the baby wasn't hers and hadn't been fed, it seemed remarkably healthy and content.
The river journey was a bit too convenient, and then arriving to find the soldiers quietly listening to a sing song with no guards posted and no challenge as to whether this stranger might be a spy felt rather implausible. Plus he never seemed to find that he needed something he had lost - it all just worked out.
Would men about to go over the top really show so little interest in someone telling them that the attack had been cancelled?
And then the big dash across no mans land. Yes it was dramatic, but as he was already too late and the first wave was being sent in anyway, why not just use the trench once it was clear (as advised) and have more chance of delivering the message to save the rest?