Video narration
ON-SCREEN A SUM formula over a column of left-aligned numbers returns 0. A small caption: "It didn't error. It just said zero." And I want to stress that "quietly". If you try to add up a column of text-numbers, Excel doesn't throw a tantrum. It just calmly returns zero, or skips them, as if they weren't there. No error. No warning. Just a wrong number that looks exactly like a right number. This is how bad decisions get made in real companies, and it is almost always caused by data that was pasted in from a website or exported from some ancient system that wrapped everything in text. How do you fix it? The quick way is to select the column, and use the "Convert to Number" option that appears in the little warning icon, or paste the column into a fresh column with "Paste Special → Values". But the habit I actually want you to build is simpler: whenever you open a file you didn't create, glance at the alignment. Three seconds. It will save you an afternoon at least once in your career.