When Beau Robbinson did his elbow last year for the Reds it was much the same from Chris Jack possibly? On the replay it looked real suspect, like he was intentionally trying to damage Beau's elbow. Nothing came of it, but I always thought it was worth another look. Can't seem to find a video of it though.
This sort of play is just appalling and needs to be stamped out, though when it happens it's rarely noticed as it's the sort of thing that can happen in a ruck I guess. In Beau's case, the offending player is behind him so when his arm gets wrenched backwards I can only assume he thinks it's unintentional, but on the replay you see the offending player grab his arm and simply hyper extend his elbow.