Since the iron door was added, it didn't have a knob texture (neither in block or item variants), as it was made this way to indicate the player that iron doors cannot be open with the hands and it has to be opened in other ways (redstone).

But after the Texture Update, iron doors were given a knob texture on them (presumably accidentally, as it wasn't announced in its changelog).

At first it seems right, because it is consistent with other doors, until you notice that the iron door doesn't work like others. I'm not sure it this can be considered a bug since there were no other reports of it.