Did this last night.<p>Wife was having trouble with a financials spreadsheet from work, and asked me to help out. I took a look, and although I didn't know the actual content, it was clear to me what the <i>kind</i> of error must be, and how to correctly design a spreadsheet so that such errors are easily identifiable.<p>She said she didn't know that the incoming data might be problematic; I replied that one must always treat <i>all</i> inputs as untrusted. She said that XYZ gave it to her; I said be defensive against bad data even if <i>I</i> give it to you.<p>Anyway... the whole thing turned into a conflict, when I could have just said "I'm glad I could help you this time".