The author details their troubleshooting journey while upgrading a SvelteKit app from Svelte 4 to Svelte 5. After successfully running the app locally, they encountered a critical issue on their staging server where no pages would load. The debugging process highlighted several differences between the local development environment and the production setup, particularly the use of Docker and various dependencies. Key suspects included the Docker environment, the Sentry error reporting tool, and the Node Adapter's behavior. Upon investigating, the author discovered that the error stemmed from an internal Node dependency called "undici," specifically related to a method called `Response.clone()`. The issue revolved around JavaScript's private class fields, which the Proxy class couldn't access, complicating the debugging process. The author initially suspected Sentry due to its role in error handling, but removing it didn't solve the problem, leading them to reconsider the implications of running the app in Docker. The investigation deepened as the author looked into how proxies work in JavaScript. They crafted additional code to track where the Proxy class was being instantiated, focusing on instances of the Response class from undici. This approach aimed to identify which part of the code was creating the proxy that led to the issue. The article provides a detailed account of this debugging adventure, blending technical insights with a narrative style that reflects the challenges of modern web development.