It is remarkable that among the 1165 errata corrected under my auspices, only several do I regard as true errors in physics. An example is Volume II, page 5-9, which now says “. . . no static distribution of charges inside a closed grounded conductor can produce any [electric] fields outside” (the word grounded was omitted in previous editions). This error was pointed out to Feynman by a number of readers, including Beulah Elizabeth Cox, a student at The College of William and Mary, who had relied on Feynman’s erroneous passage in an exam. To Ms. Cox, Feynman wrote in 1975,* “Your instructor was right not to give you any points, for your answer was wrong, as he demonstrated using Gauss’s law. You should, in science, believe logic and arguments, carefully drawn, and not authorities. You also read the book correctly and understood it. I made a mistake, so the book is wrong. I probably was thinking of a grounded conducting sphere, or else of the fact that moving the charges around in different places inside does not affect things on the outside. I am not sure how I did it, but I goofed. And you goofed, too, for believing me.”
BasicFeynman.com: Basic Books Celebrates the Life and Work of Richard P. Feynman
This error was the subject of a thread a few years ago. IIRC, JD argued that the ungrounded cage (shell) would "hide" the distribution of the charge, and, therefore, no error.
bc 's memory NOT infallible.
p.s. also, IIRC, I had read that section of the Lectures and puzzled by it. JD's explanation satisfied me then.