I've used Shimano Sora shifters on and off road for three years and the biggest hang-up I have with them is my thumb hanging up on the cable housing when shifting, especially when I'm off-road cyclocross style.
Must be some others out there to whom this happens. Or maybe I'm spastic -- but I doubt it -- I think it's the shifters!
Anyway, here's the solution I came up with. I used to hang up 50% of the time when shifting in off-road turbulence, now it's about 0% of the time. No loss of function as far as shifting goes, in case you're wondering, and I've fully field tested this modification:
I forgot to mention there's another benefit, which is that (following the mod) you can reach in and pull the lever from the inboard side. This is especially useful on the left shifter, where you have to pull the lever through a wide radius, and it's just easier to reach in from the inboard side and kind of scoop the lever, using your forearm as additional leverage off the flat part of your bar if you want. Helpful too when your hand is going numb.
I'll drop a photo of that application into this thread if the friggin' megabyte ceiling on photo attachments will let me. . . .