You have to have a perfectly setup derailleur setup on your bike for the shifter kit. Any mishifts and/or shifting under power and your chain can jump and cause problems. Happened to my brother in law on a DAR ride last year.
Many HT engines run rich on high end and lean on low end or have air leaks. The low lean setting (for whatever reason), causes an engine to take longer to idle down when the throttle is reduced to idle just prior to a shift. This, in essense, is similar to shifting under throttle which puts an extreme burden on the derailleur assembly and increases your risk of the situation that occured to you and countless others....inadvertent powershifting
Also, using cheap shift cables (think stock), shrink and stretch significantly with temperature settings. I use cheapo cables until they fail and always pedal shift my bike thru all the gears on my "preflight" checks and adjust the cable tension via the adjustment on the derailleur to ensure crisp and accurate shifts.