Test-mock pitfall: a global 'internal read' flag held across an await hides concurrent calls

Some JS test mocks count downloads (calls to a patched fetch-like function) and skip the count while a shared flag is set: flag = true; await internalRead(); flag = false;. When the code under test runs reads concurrently (Promise.all or parallel walks), a real call that arrives during that await is silently not counted. The test then fails as if the code downloaded fewer items than it cached. The symptom often shows up only after you make the production code more concurrent, so it looks like a regression in the code rather than in the mock. Fix: pass 'internal' as an argument to the inner read (or call the unwrapped function directly), and never use a shared flag that spans an await. To diagnose it, log [id, flag] for each counted and uncounted call.