With actors it's easy to end up either with circular deadlock (A sends a message to B and waits for a response, B sends to C, C sends to A, stop), and/or you can end up with backpressure problems where different parts of the system run at different speeds and overflow their message queues. All these issues have a habit of showing up only in certain timing conditions or if things only happen in very specific orderings, especially because different parts of the system can post messages to a central actor in different orders depending on execution speed.