Why do models (sometimes) crash? 
  
The most common reason for a crash is radio interference, if you believe what unfortunate pilots say immediately after the crash. But of course you should not believe what they say at a time like that.... 
 
I have studied my own 23 24 crashes and made some statistics, after some objective thinking
Updated 2004-07-12 
 
Crashes
Caused by
Typical reason
3
Pilot not trained
Tried to learn to fly alone, on a too difficult airplane
10 11
Pilot error
Flying too low or making a too difficult manoevre at low height, stall while turning at low height and with low speed. Two pilots trying to fly on the same frequency..... 
(I had the frequency peg!). Stupid manouvre!
2
Difficult type of airplane
Small and fast, only single channel control
7
Technical error
Interference from electric motor (AM radio), wing broke in middle because of hi-g manoeuvre, short radio range because of broken X-tal from previous crash, bad connectors, wire broken at solder joint, interference from cables in ground (only noticed when flying low…). Glitches from brushless outrunner interfered with receiver, making unmanageable control inputs...
1
Inadequate design and construction
An extremely bad ARF
 
65% pilot error or pilot dependent (should never have flown...) 
30% technical errors, including one case of  interference 
5% inadequate model