Debug without guessing

Help me debug this systematically. No fixes until the cause is proven.

What I expected: <the behaviour that should happen>
What actually happens: <the behaviour, exactly>
The error, in full: <paste the complete message and stack trace>
Steps that trigger it: <exact clicks or inputs>
What I've already tried: <list, including things that didn't help>
Relevant code: <paste the files or functions involved>

Follow this order and do not skip steps:
1. Restate the bug in one sentence and confirm the smallest way to reproduce it.
2. List 3–5 possible causes, ranked by likelihood, each with the one observation that would confirm or rule it out.
3. Tell me exactly what to check or log to test the top cause. Wait for my result.
4. Once a cause is confirmed by evidence, explain it in plain language: why this code produces this behaviour.
5. Propose the smallest fix that addresses the cause, not the symptom. Show the diff.
6. Say how to verify the fix, and what else could be affected by it.

If at any point you are guessing, say "I'm guessing" and propose a test instead of a fix.