The failure with no error message.

The worst failure is the quiet one. No error, just a step that never happened.

You have met the loud kinds in this course. The wrong fact. The useless answer. Both told you something was off. This one doesn't. You ask for three things, two come back, and nothing anywhere says so.

Today you get the habits that make quiet failures visible. One line before you run, one glance after. That's most of it.