Here's the comment in the AMA.
according to my grandfather, he was mid-march between camps when they were liberated from American soldiers. "We marched westward through the Bravarian countryside and in the daytime American planes flew very low so I could see the pilots face behind his mask. They were machine-gunning us mercilessly. We waved at them with bed sheets and they could see our striped clothes. The Americans killed more inmates in the last 8 days than the Germans did." He says the last camp he was incarcerated in was called Flossenburg.
The worst incident of this sort was the sinking by RAF fighter-bombers of the SS Cap Arcona which was laden with concentration-camp prisoners. At least 4000 prisoners were killed either in the attack or by the Germans as they tried to escape.
On a smaller scale Daniel Goldhagen mentions a similar event to that in the AMA (Hitler's Willing Executioners, chapter 13) though here the American plane actually killed an SS woman, and her colleagues then killed many of the Jewish prisoners in revenge. I expect things like this happened quite frequently; as someone says in the comments to the AMA at 400mph a column of men accompanied by miltary vehicles would look like a legitimate target.