Clipless pedals (safety release)
In my humble experience with extensive crashing during mountain biking, including a lot of over-the-bar experiences and a few cases when I broke bones, before you hit the dirt, clipless pedals magically disengage themselves. In reality, being clipped is your least problem.

You really need to slowly stop the bike and then fall on a side to still remain being clipped in.

P.S. Just read the comments and discovered that it is not just me who observed magical disengagement of clipless pedals in the course of a crash.