The bad-tempered clavier
I have recently been listening to DJ J.S.Bach's bitchin' dance album "The Well-Tempered Clavier". And I looked it up on Wikipedia, where I found out that grown adults spend their professional lives argufying and theorising about what the curlicues in DJ Johann's hand-writing can tell us about the precise frequencies at which he tuned his instruments.
What none of them seem to have realised is that Bach had no way of knowing what those frequencies were and that while the knowledge existed for contemporary natural philosophers to have taken a stab at figuring out the relative frequencies (eg that this C is twice the frequency of that C) they would not have been able to make usefully accurate measurements of the one or two Hz differences - which are down in the quarter of a percent range - that these stupid academics are arguing about.
In fact, I can't really imagine a Baroque-era experimenter getting results more accurate than within 5%. And 5% is as near as damnit a semitone.
Grrr