The task model, synchronisation context and deadlocks, cancellation, and parallelism choices.
3 questions in .NET & C#.
A CancellationToken in C# is cooperative cancellation, not a thread abort. Pass it through async calls, check it around long CPU loops, and define whether cancelled work leaves durable side effects behind.
One exception. WhenAll's task faults with an AggregateException holding every failure, but await rethrows only the first inner exception, so recovering the rest means keeping the WhenAll task in a variable and reading its Exception property.
Blocking on async code with .Result or .Wait can deadlock when the awaited continuation needs the same SynchronizationContext that is blocked. In ASP.NET Core the common failure is thread-pool starvation; the fix is async all the way up.