Best of Code Review 2016 - Diplomat category

Question

Please post your nominations for the Best of Code Review 2016 - Diplomat category.

Tough advice in an answer, delivered in the most tactful manner.

In your nomination post, please make sure to include a link to the nominated answer. Include a short explanation of what makes that post worthy of being nominated in this category, and why it should win over the others.

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_{Small characters at the bottom: Only one nomination per post. Nominated answers must have a creation date in 2016. Downvotes don't count. Santa reserves the right to award the top-voted nominee a special bounty as a token of appreciation on behalf of the Code Review community.}

Answer 1

Nominating this answer from Gareth Rees to the question Google Foobar Challenge: Lucky Triples for the lengthy details and teaching nature of an answer that boils down to "your code doesn't work as expected, try this instead".