Hi billybones, did you put the two together in one injection? It depends on how much iron was used (b12 not so much of a worry but iron is a danger if overused or injected into the wrong spot) yes can cause damage if not used carefully according to strict directions on the bottle.
(Toxic levels of iron can cause damage to the stomach and intestinal lining, also cause severe liver damage and heart damage.)

Dogs are more easily affected by iron overdoses than people because they do not have a way to excrete excessive iron from their bodies.