Those of us in the last two category have had to fight an enemy most foul: the double strum. When you strum the bar, it will play the note, but then spring back and play a second note on the rebound. The harder you strum, the harder it bounces back: it's especially bad on the up-strum. Once you get into hard mode, it's a near-impossibility to get through without strumming up and down, and passages that are played perfectly will get you booed off the stage.
The problem is this: the strum bar sits right on top of two switches, one for the up strum and one for down strum. The only thing restraining it from playing the second "ghost" note is the weak spring inside the switch itself. We need to add some padding to the switches that will stop the bar from pressing on the opposite switch on the rebound without interfering with a normal strum.