I'm not following, how could an error with Future[Either[A,B]] disappear compared to synchronous logic of Either[A,B]? Our code base is the same for sync and as

I'm not following, how could an error with Future[Either[A,B]] disappear compared to synchronous logic of Either[A,B]? Our code base is the same for sync and async logic and error handling. One thing that doesn't work is Anders Hejlsbergs method of letting unchecked exceptions bubble up, but exceptions haven't been a good idea for business code anyway.