I'm only really an RAF guy, so I'll let someone else speak for the USAAF/Luftwaffe/Eastern Front, but the closest I can think of matching this scenario would be the 1945 Bomber Command raid on Dresden - a city which was at the time sheltering a significant number of refugees from other German cities. Refugees being caught up in such an attack is quite different from merely opening fire on a column of pedestrians, but that was very much NOT the done thing amongst Western fighter pilots and ground attack, who had a deeply engrained sense of honour and an oddly anachronistic chivalry.

I have not really read anything that would 100% prove that. However, I would not be surprised if that did happen in the Eastern Front. I can tell you something similar that happened.

A German unit was defending a train as it was loading German wounded onto it. The Russians were closing in, and they were within 500m of the train. The train had red crosses plastered all over the top and sides of the cars. The Germans held out for as long as they could, and the train succeeded in getting away due to them sacrificing the slower wounded that could not board the train in time. 3 minutes later, a pair of Il-2 Sturmoviks roared over the Germans headed towards the train that just left. They fired their rockets and bombs on the red cross train carrying the wounded, killing most of them on board as the train derailed.