I think a lot of the false-positive cases where the screen gets locked are acceptable in context.

I mean, most people dropping their phone will be too glad/devastated that the device did/didn't escape harm to bother being annoyed that they have to unlock the screen again.

A drop registers as no acceleration while in freefall, and then a sharp spike when it hits the ground. This was counter-intuitive to me when I first figured out how to display my phone's accelerometer readouts, but makes sense.