Describe a time your report showed something unexpected or a stakeholder disputed the numbers.

This happens to everyone in BI, and how you handle it is the whole question.

• Check your own work first, quickly and without defensiveness. Trace the number back through the measure, the model relationships, and the source. A surprising number of disputes turn out to be a filter context issue or a many-to-many relationship producing inflated totals.
• Find where the two numbers diverge. If someone's spreadsheet disagrees with your report, reconcile at a granular level rather than arguing about the total. The gap is almost always a definition difference — a different date field, an excluded category, or a currency conversion.
• Be willing to be wrong publicly. If your report was incorrect, say so immediately and fix it. Trying to defend a wrong number costs far more credibility than the error did.
• If the report is right and the news is bad, present the evidence calmly and let people react. Do not soften a genuine finding.

Note: The systemic fix is worth adding — documented measure definitions and a data dictionary, so "what does this mean" is answered once rather than every time.