in MSE we find that Axel Harnack in 1885 proved that the interval [0,1] can be covered by a countable number of small intervals such that a countable number of intervals remains in the complement. Nobody thought that Harnack's proof was flawed till Emile Borel appeared on scene. He proved that the complement of a countable union of intervals can be an uncountable union.
Harnack, a pioneer of measure theory, could not react because he passed away in 1888. But he became professor in 1876 and may have had some students.
My question: Has any mathematician defended his proof?