We've had a couple users run into issues in the past trying to use a client with multiple tokio runtime instances, for example https://github.com/mongodb/mongo-rust-driver/issues/615.

It would be good for us to document this somewhere in the manual. Maybe in our client usage guide we could say something like "Clients can only be used with a single runtime instance. If you are using tokio::test you'll need to create a new client for each test. If you want to reuse a client across tests you should use the plain test attribute and share a static runtime across test cases."