A toot by @csepp astutely phrased a UI design rule as: "state should be visible".

That reminded me of one of the best (worst) examples of this.

In Excel, when editing a cell, the editor can have two different modes:

• In the first mode, you can edit the text, but pressing arrow keys move to a different cell
• Alternatively, there is a mode where the arrow keys move the cursor in the cell

You switch between the two modes by typing F2, but there is zero indication of which mode you're in.

Have you ever wanted to move the cursor in the input field and instead you moved the cell? Well, there's your reason.

Why on earth Microsoft has releases an uncountable number of versions of Excel over what, 30 years? and never even thought about adding perhaps a little outline around the cell when in one of the modes is beyond me.