Hey Drx.
Does it happen in all downshifts or just a certain specific gear, like 3rd-to-2nd gear?
About how often would you say it happens?
Has the bike ever been down on its left side? If so, did the shift lever make contact with the road?
Is your clutch cable adjusted properly? How wide is the gap between your clutch lever and clutch-lever housing:
How about when you lightly (barely) touch the lever? (You don't have to measure the gap with a feeler gauge--is it about the thickness of a nickle? About a half-inch?
Also, never rest your foot on the shift lever. And when you shift, don't do it gradually, but with a good firm difinitive motion. Not stomping the pedal, but not feathering it either.
Let us know on the questions.
-CCinC