[The Squatter of Amon Rûdh]

I think that this issue of the Last Bridge really demonstrates what a mistake it was to try to re-work The Hobbit to fit The Lord of the Rings. The real pity is that Tolkien had already made enough references to the Silmarillion material to make it virtually impossible to use it in another book without setting it in the reality of The Hobbit. This doesn't absolve him from this alteration, though: the discovery of the trolls is amusing and provides a link to the earlier work, but it was in no way necessary to add that incident in order to combine the two stories. It looks as though the inclusion of the scene in the Trollshaws in the later work, and associated re-writing of The Hobbit to bring it in line with the new material was an act of pure self-indulgence (possibly with half a mind on the effect it would have on those who had read the earlier book). I quite like the scene with the petrified trolls, so for me this isn't too much of a problem; but it's certainly an obvious case of Tolkien being unable to resist re-writing material that didn't need to be changed.

As for the shadow from The Ring Goes South, I'm intrigued by this error. It's an easy one to miss, as evinced by the fact that I'd never really thought about it before; and it isn't essential to the plot other than to add a sense of foreboding and general threat at that moment. Although we might be able to excuse Tolkien for missing this small slip in a work as large as The Lord of the Rings it does serve to remind us that he wasn't as infallible as some of us might sometimes think him.

Regarding The New Shadow, personally I feel that any mistakes in works that Tolkien never submitted for publication are irrelevant. He lost interest in The New Shadow before he had even worked out how it was to end, so for me it stands only as an example of an idea that came to him. It is the nature of rough material to be unfinished and to contain errors, particularly with an author like Tolkien, who was in the habit of writing early drafts in a great hurry and then re-writing them several times.

It may not be a continuity error, but the classic example of a Tolkienian slip is the passage in The Bridge of Khazad-dûm in which the Balrog is described. Having quite legitimately set up the picture of a creature wreathed in a shadow that looks like huge wings, he then forgets that he was using a simile and refers to 'its wings'. As Mister Underhill pointed out when he mentioned this in another thread, this one slip has caused so much fruitless argumentation and speculation that it's spawned the joke of referring to solutionless Tolkien debate as 'balrog-winging'.