Asynchrony
Defects in how methods are declared and composed with async/await, where the asynchronous control flow escapes the caller’s ability to observe it. The signature looks ordinary, but the returned Task — or the absence of one — changes who is responsible for the work and its failures.
The dominant case is the async void method: it returns no awaitable, so the caller cannot await completion or catch what it throws, and an exception is re-raised on the synchronization context where it typically tears down the process. Outside event handlers, an async Task return is almost always the correct shape.