Challenge 2: Assuming GIMP's Direct Adjustment Was Non-Destructive — Solution Walkthrough

The problem they're likely to run into:

Per this chapter's own warning box, applying a color adjustment
directly to a layer in GIMP does not automatically behave
non-destructively the way Photoshop's Adjustment Layers do by default
— the adjustment permanently alters the layer's own pixel data unless
a deliberately non-destructive approach (a duplicate layer, or a
genuinely non-destructive GEGL-based filter) was used instead. After
several more edits have been made on top of that adjustment, there is
no simple, direct way to isolate and undo just that one earlier color
change without also undoing every edit made after it.

Why this specifically catches this user off guard:

The user's assumption came directly from experience with Photoshop's
own default behavior, where Adjustment Layers are automatically
non-destructive and easily removable at any time regardless of what's
been done afterward. Applying that same assumption to GIMP's own
direct adjustment mechanism led to a real, avoidable problem, since
GIMP's own equivalent behavior isn't automatic the same way.

What should have been done instead:

Per this chapter's own material, the user needed to deliberately
choose a non-destructive path in GIMP — working on a duplicated layer,
or specifically using GIMP's own non-destructive GEGL-based filters —
rather than assuming any adjustment applied would be automatically as
easy to isolate and undo later as it would have been in Photoshop.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise directly applies this chapter's own warning box to a
realistic, painful consequence of the exact assumption the warning box
was written to prevent.