Concurrent independent operations
await Promise.all([fetchA(), fetchB(), fetchC()]) in JS, asyncio.gather in Python, structured concurrency in Kotlin. Sequence requests in parallel when their inputs are independent. Easy 3x speedup; commonly missed.
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Cancellation
Long async operations need to be cancellable: timeout, user cancels, parent cancels. Use AbortSignal (JS), CancellationToken (.NET), structured concurrency scopes (Kotlin, Java). Don't ignore cancellation — leaks zombie operations.
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Error handling
Errors in async operations propagate through await. Uncaught rejected promises crash the process (in modern Node). Test the error paths; they're harder to reproduce than sync errors.