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This problem is easily fixed by having Harry go first.
That's not exactly a flaw. Maybe Harry just isn't quite a logician and is trying to deduce the colors from his own marbles and label only.
If we do not know the level of skill of the characters in the puzzle, then these puzzles become unsolvable. There is a precedent set by "5 Pirates" and "Black and White Hats" and others that if a character can figure out the problem he/she will. Unless we are told that characters lie or are imperfect, we have to assume that they are perfect logicians else all problems become unsolvable. Otherwise the line "I don't know" by any character becomes a useless piece of information.
And in this case, if you take away the line "but he could not tell the color of the remaining marble" the problem is unsolvable. This line is an irrelevant piece of information unless you know how skilled of a logician he is.
While this is an interesting problem to work through, there is a flaw in the puzzle. Usually in puzzles we are to assume that if the person cannot figure something out (Harry does not know the color of his last marble) it is because this person has not been given enough information to figure it out. However, in this case Harry does have enough information to know his last marble. This is clear, because no additional information is revealed after Harry does not know the color of his last marble, yet I (the reader of the puzzle) and Sally both have enough information to figure out what Harry's last marble is.
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