After writing this it occured to me that the first Christopher Reeve Superman movie does the same thing (only without the mystic energy source bit). Sure Superman's dad tuts a lot about not turning back time, but its not clear that anything bad happens as a result and again this was presumably made with the intention that there might be sequels so its a bit silly to make your central character quite that powerful. Though of course you could, and many have, argue that Superman is so ridiculously powerful from the off that anything else is neither here nor there in terms of plot problems.