You can loose server side events under the following conditions:

• long-polling: between reconnection, servers side events may happens and if they aren’t persisted, those events will never reach your client.
• websocket: Websocket are new and most if not all firewall will close them after some X idle times. Again, all server sides events will be lost
• http-streaming: Some proxy really don’t like the http-streaming technique, and will close it right away. Again, possibility to loose server sides events.
• Unexpected network outage: the connection can also be closed by something between your browser and server.

Je pense qu'il ment pour WebSockets, Hixie n'aurait pas pu laisser passer une telle déficience, il est infaillible et tellement pragmatique.