Compare dev and production

Works in development, broken in production.
Symptom: <describe>. Production error: <paste from host logs>.
Stack: <stack>. Host: <host>.

Go through what differs and tell me which could produce this exact
error:
- Environment variables set locally but not on the host.
- File path case sensitivity.
- Working directory and relative paths.
- Runtime version.
- Build step outputs missing or stale.
- HTTPS, secure cookies, mixed content.
- Database version, or a different database entirely.
- Timeouts and memory limits the host imposes.
- Network egress rules.

For each plausible one: exactly how do I check it on the host?