I have a user that reported an issue with Tomcat8 + Ubuntu, where the same websocket application would work without any issue with Tomcat8 + Window:

   https://groups.google.com/forum/#!topic/atmosphere-framework/-exX6IKdB9s

Several users also reported similar issue with Tomcat 8, not only on Ubuntu, but it is extremely hard to reproduce. But the user above was able to reproduce it 100% of the time. The observed exception is

06:38:07.568 ERROR [http-nio-8080-exec-2] o.a.c.AtmosphereFramework [AtmosphereFramework.java:1822] AtmosphereFramework exception
java.lang.IllegalStateException: Not supported.
	at org.apache.catalina.connector.Request.startAsync(Request.java:1604) ~[catalina.jar:8.0.5]
	at org.apache.catalina.connector.RequestFacade.startAsync(RequestFacade.java:1037) ~[catalina.jar:8.0.5]
	at org.atmosphere.cpr.AtmosphereRequest.startAsync(AtmosphereRequest.java:692) ~[AtmosphereRequest.class:2.1.4]
	at org.atmosphere.container.Servlet30CometSupport.suspend(Servlet30CometSupport.java:93) ~[Servlet30CometSupport.class:2.1.4]
	at org.atmosphere.container.Servlet30CometSupport.service(Servlet30CometSupport.java:68) ~[Servlet30CometSupport.class:2.1.4]
	at org.atmosphere.cpr.AtmosphereFramework.doCometSupport(AtmosphereFramework.java:1801) ~[AtmosphereFramework.class:2.1.4]
	at org.atmosphere.cpr.AtmosphereServlet.doPost(AtmosphereServlet.java:210) [AtmosphereServlet.class:2.1.4]
	at org.atmosphere.cpr.AtmosphereServlet.doGet(AtmosphereServlet.java:196) [AtmosphereServlet.class:2.1.4]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:618) [servlet-api.jar:na]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:725) [servlet-api.jar:na]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:301) [catalina.jar:8.0.5]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [catalina.jar:8.0.5]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-websocket.jar:8.0.5]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239) [catalina.jar:8.0.5]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) [catalina.jar:8.0.5]
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219) [catalina.jar:8.0.5]
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106) [catalina.jar:8.0.5]

Note that <async-supported>true</async-supported> is defined in web.xml. The sample used to reproduce the issue is:

   http://goo.gl/AxmXf9

Now when using Tomcat 7 everything works as expected, both using native and jsr356 implementation for websocket. I'm not sure if that is caused by the websocket implementation, but I must say it always happens when there is a mix or websocket and http/comet connections occuring. 

The sample above is configured to use websocket when available, and fallback to long-polling if websocket aren't supported. That's the case when IE 9 fails.

Sorry if the issue is vague, but it is extremely hard to reproduce.