Interesting discussion so far, everyone! Thanks to all for your contributions. From the wording in the quote given in Post #1 Carter seemed to have taken it as a given that Frodo did commit wrongs, and was punished as a result. I'm sure he would be interested to see that the majority of posters here disagree with him, and that we are all ready to spring to Frodo's defence!
Frodo was, and still remains, my favourite character in the book, and I personally think that if he did do wrong, it was minor and forgiveable. After all, the quest was fulfilled because of his strong will and remarkable resistance to the Ring, with the help of Samwise, and as a direct result (as has been mentioned earlier) of the pity and mercy with which he treated Sméagol. I had never conceived of Frodo as guilty of any real mistakes until reading the Carter quotation.
Bêthberry, the "mistake of fact" which most readily springs to mind is that Carter referred to Éowyn as Théoden's daughter. I can't recall any others, but they were all minor errors in reading that seemed to give the overall impression that Carter may have only read the book once or twice, and not completely soaked in all the details yet.
Some have mentioned that the term "sin" or "sinners" is not applicable in Middle Earth. I agree that it would be out of place within the text, which was purged of
almost all references to religion. But can we not discuss the book using such terms? Isn't it, after all, "consciously" Catholic in the revision?
I can't help but feel that by having Frodo fail at the last and give in to the temptation of the Ring, Tolkien was trying to say that even great heroes are not without sin (although he still upheld such chivalric heroes as Aragorn, Gandalf and Faramir).
What do people think of the other folk that I mentioned: Boromir, Fëanor, Saruman, Wormtongue, Thingol. Did Tolkien dish out "just desserts" to these characters as a
consequence (thanks,
Holbytlass!) of their ignoble actions?