Hi
I looked at the FAQ list and this is a more hands-on subject, but it's still a real FAQ:
Time and again new users ask in the questions forum * why they can't get the inpainting brush tool (and others) to work. The answer is: you have a wrong layer selected.
I'm quite sure the question arises so often because some adjustments (like levels) create a correction layer without the user noticing. So it also affects people who aren't actively using layers yet.
Feature request: could you add a warning when people try to use inpainting on a wrong layer?
* and this are the ones bothering to register in the forum and start a thread