Challenge 2: Accidentally Damaging the Image While Trying to Paint the Mask — Solution Walkthrough

The most likely mistake:

The user was painting on the layer's own image thumbnail rather than
the mask thumbnail — exactly the common beginner mistake this chapter
names directly. Painting black directly onto the image itself with
the Brush tool actually paints black pixels permanently into the
image data, which is genuinely destructive, unlike painting black on
a mask, which only hides pixels without altering them.

Why this produces "permanent damage" specifically:

Per this chapter's own distinction between masking and erasing (or, in
this case, directly painting over), any tool applied straight to the
image's own pixel data changes that data permanently — there's no
mask sitting between the brush and the actual image to intercept the
paint and simply hide content instead of altering it. The user
intended a non-destructive hide, but because the wrong thumbnail was
active, they performed a fully destructive paint operation instead.

How to avoid this in the future:

Per this chapter's own material, always confirm which thumbnail
currently has the highlighted border — indicating it's the active
target for brush strokes — before painting. If the mask thumbnail
isn't clearly highlighted, clicking it first ensures the brush
actually paints onto the mask rather than the image itself.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise directly applies this chapter's own "common beginner
mistake" warning to a realistic destructive-editing scenario,
correctly identifying the wrong-thumbnail mistake as the root cause
rather than a tool malfunction.