Barring all other problems, there's no cognitive overhead when it's clearly labeled. You simply read what it says.
What people have to understand is that it doesn't matter how clearly you think you're labeling it, if it breaks the user expectation, it's not clear at all - and an experienced user reads even less, which is the case of a pilot.
Once the pilot develops an habit of, e.g., switching the vertical speed display from feet/s to degrees during final approach (and that may have been what happened), and one day he forgets to switch, or the mode button malfunctions, or double clicks by mistake... there you have a recipe for disaster.