"Oh Shit" moments...

Hang on, does that mean that my Garmin cadence metre gets it wrong when it clocks me at 200rpm descending on my geared bike?

What kind of descent would you need to do 200rpm on if you were on gears?
It's the sort of figure sprinters hit when doing specific training, sprints at that cadence will last no more than 10 seconds or so.

Highly unlikely, inefficient and pretty unnecessary for most roadies to be doing it.