Another one that has always interested me.
Heuer used Lemania 5100s in design variants that didn't use the 24-hr dial (like a 5012).
At the same time they came up with the 510.500/12 variant with a 24-hr dial but actually using the 5012 instead, the only watch anyone knows that has such a sub-dial on the 5012 movement.
So the mystery is which person was mucking up the parts bin inventory which required the simplification of 5100 watches and the later complication of 5012s, when swapping movement with no customisation would have worked just as well.....