touchy shifter
 

I've been noticing lately that on my Cannondale Trail 5, if I brush my thumb against the shifter lever it will almost always change gears. Is there any adjustment on this?

That's like the guy who goes to the doctor and says his arm hurts every time he twists his wrist. Solution ---"don't wist your wrist".

Depending on your shifter, this may be a sign on non-fixable wear, or a simple adjustment. If you provide specifics, you'll get a specific answer, instead of old jokes.