I saw a brand new dirt bike ridden by a beginner completely explode from doing that a few years ago. I believe it was a Honda XR250. Seems the rider was going pretty fast in top gear, and downshifted it all the way to first gear, and let the clutch out, scattering engine and transmission parts all over the place. The sudden shock cracked the case in several places where the crank and trans shaft bearings were mounted. One of the shafts was sticking way out through a large hole in the case.
When I first started teaching my kids how to ride MANY years ago, I drilled it into them to never downshift more than once without letting the clutch out. After a few years they learned how much they needed to downshift depending on their speed and the gear they were in. Two kids, they went through 6 dirt bikes, as they got older, still have the last 2, and never damaged a single one, or themselves. Both girls, today they can ride as good as any guy.