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.
Step 1: Disassembly
Once all the screws have been removed, you should be able to pull the back cover off and reach the delicious innards. Make sure you note how the strap buttons are attached and keep track of them.
The offending switches are under the central and largest circuit board. First, remove the three screws holding the analog thumb controller. Remove the four corner screws from the strum bar circuit board and lift it up (with the thumb controller board and neck connector).
Step 2: Padding the switches
Take some double-sided mounting tape and cut out a hole for the switch actuator, making sure that it is not blocked or impeded in any way. I used an x-acto knife to make the cuts. Make sure that yours is new and sharp. The second picture below shows what the cuts will look like when the x-acto knife blade is worn and dull.
Cut some foam to size and adhere to the mounting tape. Make sure that the foam comes above the switch actuator, with about 1/4" to 3/8" clearance. It needs to provide just the right amount of resistance to strum bar rebound without interfering with normal gameplay. I used foam from a microphone shipping container and a worn and dull x-acto knife. Once again, I recommend new and sharp.
Reassembly, as usual, is the reverse of disassembly. Be sure to include the strap buttons in the first reassembly to avoid a second disassembly and reassembly.