I periodically see designers, with serious (albeit cheerful) faces, boldly clicking the 'round button' in Layers to 'select everything on the layer.'

And while the button indeed 'selects' everything on a layer, it also selects the layer itself — targets it really, for applying a layer-specific appearance.

Take a look at this GIF attached.
If I select Layer 2 (with the green squares) like this, and then apply the Round Corners effect, all the new squares in the layer will have round corners.
Sure, everything is visible in the Appearance palette.
Sure, everything is written in the pop-up hints.

The round ones are for appearance. We need to click to the right of the circle instead, where the square markers are drawn — these are there to mark selection... AND allow selection.

It’s a very common mistake (like confusing windshield wipers and turn signals)...
And I think I know why it happens. I’m convinced that it is psychologically easier to click a circle — that looks more like a button, — than an empty space where a square appears (when clicked).
Recently some accessibility changes were introduced into Ai, and now things we can click in most panels gets highlighted slightly on hover — but not these square-to-be spots! Even when there are square markers there!

So please make these more pronounced.

I love appearance dearly, but selecting all in a layer / sublayer / group is a far more common procedure users do. And discovering something called 'appearance' was the source of many problems with art’s colors is not a good way to meet this power.