Flight Control Failures

Following the thread on landing with one spoiler, I wonder what
proportion of landing accidents result from mechanical failure or
jamming of primary flight controls and spoilers/dive brakes. I
personally know of a couple of failures - one spoiler handle sheared
off as the pilot did his pre-landing checks and he landed safely
off-field and another had a pencil jam in controls, limiting control
movement, but again landed safely.

With the exception of incorrect assembly, I have to believe that
asymmetric control failure or jamming must be an extremely rare cause
of landing accidents compared to the more common reasons we all know.

Any actual data or good anecdotes?