Pick the symptom you're seeing — each page reproduces the bug on a live sheet and shows you why it happens.
Lookups
Why is my VLOOKUP returning #N/A?
The value is right there in the sheet, but the search walks the whole column and never finds an exact match.
Why is my VLOOKUP returning the wrong value?
No error, nothing red — but the number it brings back comes from the wrong row.
Why is my XLOOKUP returning #N/A?
The modern lookup misses too — usually for the same invisible reasons.
Totals
Why is my SUM formula returning 0?
The column is full of numbers, yet the total comes back 0 — they may not be numbers at all.
Why is my SUMIFS returning 0?
Every criterion looks right, yet nothing matches — one condition quietly excludes every row.
Why is my AVERAGEIF returning #DIV/0!?
Zero cells matched the criteria, so there is nothing to divide by.
Counting