I am sorry to hear about your dog. Unfortunately I can't think of a specific seed or type of vegetation that may have been the culprit.  Could it have been a thorn or part of a larger seed or plant that broke off?  Hedge mustard comes to mind because it has sharp seed pods, but I don't think it would be green in the fall. If you find a plant that you feel might pose risk as late summer and fall approach, please send photos directly to the email address below and I'll try to help.