I'm reading a couple of Matt Ridley's excellent books right now - Genome and Nature via Nurture. They're both terrific books - check them out.
You might recall a few years ago when Al Gore made reference to the 'extra chromosome' right. This apparently was a reference to Down's syndrome, once known as "mongolism", which leads to a characteristic appearance and mental retardation.
When I was a kid I had a science book that wrote of chromosomes and included an illustration. The caption stated that there were 24 pairs of chromosomes, but that some researchers claimed that there were only 23 pairs instead of 24 pairs.
Back to Ridley and Genome. He tells us that in 1921 a man named Theophilus Painter did the work that resulted in the 24 count. He worked with thin sections of tissue from the testicles of three men who had been castrated for insanity and "self-abuse" and counted 24 chromosomes. It was plausible enough, anyway, what with chimpanzees, orangutans and other apes having 24 too.
It took about 30 years for someone to challenge this by determining that humans had 23 chromosomes. They were right, but damage had been done in the meantime - some researchers had actually thrown out their results because they weren't willing to challenge the 24 number.
So how's that for luck? I'm guessing that our presumably involuntary tissue donors happened to possess this extra chromosome, and neither the researcher nor anyone who followed thought to challenge the result.
CORRECTION: I try to make this blog original, and that I am occasionally full of beans is not news to those who've been here before. However, someone was kind enough to correct me in the comments and let me acknowledge that here. The extra Y chromosome condition I mentioned is distinct from Down's syndrome, in which each cell has an extra copy of chromosome 21. Anyway, I thought I'd better correct it before I got a job offer from the BBC.