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Thanks! Matt Ridley discusses that also, in his book "Genome". Here's a reprise:

"For thirty years, nobody disputed this 'fact'. One group of scientists abandoned their experiments on human liver cells because they could only find twenty-three pairs of chromosomes in each cell. Another researcher invented a method of separating the chromosomes, but still he thought he saw twenty-four pairs. It was not until 1955, when an Indonesian named Joe-Hin Tjio travelled from Spain to Sweden to work with Albert Levan, that the truth dawned. Tjio and Levan, using better techniques, plainly saw twenty-three pairs. They even went back and counted twenty-three pairs in photographs in books where the caption stated that there were twenty-four pairs. There are none so blind as do not wish to see." (Matt Ridley, Genome: The Autobiography of a Species in 23 Chapters, p 23-24)

The correct number of chromosomes could have been discerned, one would think, during the almost 35 years involved in the events above. The most daunting realization is that the double heliacal form of DNA was discerned in 1953, two years before this chromosome count was corrected.