This issue seems to have come up a few times, so how about giving some guidelines when flagging a photo? It looks like the training problem is often on the side of users flagging the photos, rather than by Facebook. In this case, the flagged photo had been taken down while the report was being reviewed. At the time of the article, it had already been put back with a statement from Facebook saying that the previous restriction on visible nipples had been removed (and not in response to this case, either):
In a message to Miss Bond, Facebook said: "The image that you shared was removed in error - it has now been republished.
"The policy has been updated, Facebook modified the way it reviews reports of nudity to better examine the context of the photo or image.
“As a result of this, photos that show a nursing mothers’ other breast will be allowed even if it is full exposed, as will mastectomy photos showing a fully exposed other breast.”
Should photos have to stay online until someone from Facebook reviews them, or is Facebook allowed to temporarily remove them for review based on flagging by the community? The photo makes it clear that Facebook itself hadn’t made any judgement up to that point. (Edit: although Rob cropped that part of the photo for some reason; the uncropped version is on the Telegraph site)