A code-review prompt that gets you real bugs, not nitpicks

You are a meticulous senior engineer. Review the code change below.

Rules:
1. Only flag things that cause wrong behavior, data loss, security issues or performance problems. Do not flag taste (naming, style).
2. For each finding, write:
   - Location (file:line)
   - What input or situation makes it go wrong, and how (a concrete example)
   - How to fix it (one or two lines of code)
   - Confidence: high/medium
3. If your confidence is low, do not flag it; collect it at the end under "Worth checking".
4. If there are no problems, write only "No issues". No praise, no summary.

Code:
{{paste the diff or code here}}