title: What can be the reason of "Unable to find subscription with identifier" in Rails ActionCable?
body: <p>I'm building a messenger application using Rails 5.0.0.rc1 + ActionCable + Redis.</p>
  
  <p>I've single channel <code>ApiChannel</code> and a number of actions in it. There are some "unicast" actions -> ask for something, get something back, and "broadcast" actions -> do something, broadcast the payload to some connected clients.</p>
  
  <p>From time to time I'm getting <code>RuntimeError</code> exception from here: <a href="https://github.com/rails/rails/blob/master/actioncable/lib/action_cable/connection/subscriptions.rb#L70" rel="noreferrer">https://github.com/rails/rails/blob/master/actioncable/lib/action_cable/connection/subscriptions.rb#L70</a> <code>Unable to find subscription with identifier (...)</code>.</p>
  
  <p>What can be a reason of this? In what situation can I get such exception? I spent quite a lot of time on investigating the issue (and will continue to do so) and any hints would be greatly appreciated!</p>