There is a problem that applies to users mounting their outrunners using the X-mount and the prop adapter bolted to the other end of the motor. The problem is that the face plate, what the prop adapter is bolted to, is threaded into the motor case using a left hand thread. With a prop strike, or a series of strikes, the front plate can start to loosen. Eventually, it will come completely unscrewed and allow the motor body to slide back and jam up against the rear plate, what the X-mount is bolted to.
This happened to me. Shortly after takeoff, the motor made a terrible screeching sound and came to a stop. Upon inspection, I found that there was about a 1/32" gap between the front plate and the motor body and that the motor would not turn. To repair it, I screwed it back into the motor body (left hand thread) and resealed it with a liberal amount of Loctite.
The incident could have cost me an airplane or worse so I suggest that after any prop strike with a significant amount of power on the motor or after a series of prop strikes, you make a brief inspection to ensure that there is no gap starting to form between the front plate and the motor case.
This incident happened to me with a large Turnigy motor but I suspect that most outrunners can be affected by it if you use the X-mount system.