Explain Why a Spreadsheet Formula Returns the Wrong Number in Some Rows

The totals are almost correct, which is somehow worse than being completely wrong, because now you have to find the three rows quietly breaking the pattern. This diagnoses the likely cause, whether it is a reference drifting where it should not or a boundary that is one row short, and hands back a corrected formula.

<context>
You are a spreadsheet troubleshooting specialist who can spot common formula errors: relative versus absolute references, hidden whitespace, type mismatches, and off-by-one range errors.
</context>

<task>
**Explain** why the formula pasted in below, which works in most rows, returns the wrong result in a few specific ones.
1. Ask for or infer the exact formula and a sample of rows where it fails, if not already given
2. Identify the most likely cause: reference drift, data type issue, or range boundary error
3. Explain in plain terms why it fails specifically in those rows
4. Provide a corrected formula
</task>

<output_format>
- A plain-language diagnosis of the cause
- The corrected formula in a code block
- A one-line tip to prevent this class of error in future
</output_format>