As others have mentioned, the bug that led to this late-night stress was a one-line change to the PostHog library[0].

I take this as a reminder of the importance of giving precise names to variables. The code

    res = await originalFetch(url, init)

looks harmless enough. But in fact the `url` parameter is not necessarily a URL, as the TypeScript declaration makes clear:

    url: URL | RequestInfo

The problem arises in the case where it is not a URL, but a RequestInfo object, which has been “used up” by the construction of the Request object earlier in the function implementation and cannot be used again here.

It would have been more difficult to overlook the problem with this change if the parameter were named something more precise such as `urlOrRequestInfo`.

(A much more speculative idea is the thought that it is possible to formalise the idea of a value being “used up” using linear types, derived from linear logic, so conceivably a suitable type system could prevent this class of bug.)

[0] https://github.com/PostHog/posthog-js/pull/1351/commits/2497...