Prompt: Debugging

Use this when something fails.

Help me debug this systematically.

Goal:
<what I expected>

Actual behavior:
<what happened>

Error message:
<paste error>

Relevant code/config:
<paste minimal code/config>

What I already tried:
<list attempts>

Please respond with:
1. Most likely causes ranked by probability.
2. The smallest reproduction to create.
3. Step-by-step checks.
4. One change to try first.
5. A regression test or checklist item to prevent this later.