Challenge 2: A Visible Seam After Reordering Mirror and Subdivision Surface — Solution Walkthrough

The answer:

Per this chapter's own warning box, this is exactly the modifier-order
problem it describes. With Mirror above Subdivision Surface, the
already-mirrored, complete shape gets smoothed as one continuous
surface. Reordered, with Subdivision Surface above Mirror, each half
gets smoothed individually before being mirrored — and those two
separately-smoothed halves don't necessarily blend seamlessly once
mirrored together, leaving a visible seam right where they meet.

Why this isn't a bug or a setting mistake:

Per this chapter's own material, modifier order isn't a minor display
preference — it changes the actual result. Each modifier processes
whatever geometry it receives from the modifier above it in the stack,
so which modifier runs first genuinely changes what the next one has
to work with.

What fixes it:

Moving Mirror back above Subdivision Surface in the stack restores the
smooth, seamless result, since the smoothing then applies to the
complete, already-symmetric shape rather than to two separate halves
smoothed independently.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise checks that a modifier-order seam is correctly traced
back to the specific order in which Mirror and Subdivision Surface
were stacked, per this chapter's own explicit example.