There are basically only two kinds of moving parts on an RP: the expression pedal, and the up/down footswitches. There are a couple of ways these break down that have easy fixes.
The expression pedal can break down when the nuts that hold it in place get too tight or too loose. The fix is simple: with an appropriately sized wrench, loosen or tighten the nut as necessary.
The up-down footswitches are held in place by big paper clip clamps (no lie), and these can sometimes get loose and fall off inside the RP. It’s not all that common, but it happens. When it does, I recommend that you remove the bottom panel of the RP (using a philips head screwdriver), and then epoxy the paper clip clamp(s) in place. Just make sure not to epoxy the footswitches too.
A lot of people who buy a used RP find that the electronics aren’t working right. If that’s the case, doing a factory reset on the RP almost always cures it. Follow the instructions in the manual.
Overall, my RPs are very reliable. But these are a few things that I’ve had repeated problems with. When they happen, don’t freak out–just fix it.