Beware of Prop Hub Cracks

Tip: Be sure to remove your props and inspect them carefully for cracks from time to time; especially after a crash, or even a seemingly minor prop strike. The DJI Phantom prop below doesn't appear to have suffered significant damage aside from superficial scratches (it hit a lightning rod at high speed, and the copter recovered attitude and continued flying without much of a hiccup):

Figure 1: This yellowish line on the DJI Phantom prop hub is finer than a hair and nearly invisible.

But notice the faint yellowish line on the prop hub, even finer than a hair. It doesn't look like anything to worry about, but apply some slight tension, and it becomes clear that it's actually a crack that runs 90% of the way across the hub:

Figure 2: Applying slight tension to the prop hub reveals that the fine line is actually a severe crack.

This crack will expand over time, until it reaches all the way across the prop hub. At that point the entire prop will likely break in two during some high throttle maneuver, and the quadcopter will fall out of the sky, perhaps to be severely damaged or destroyed on impact.

So I advise to remove your props regularly, and check carefully for hairline cracks, especially in the hub area. And after a crash or prop strike, check the props before flying again, and replace any that have even small hairline cracks.