Sometimes a close results in this stack trace:

15:31:17.876 [default] [default I/O-3] ERROR o.a.container.JSR356Endpoint - java.io.IOException: io.undertow.websockets.core.WebSocketFrameCorruptedException: UT002015: Extensions not allowed but received rsv of 4
2014-04-07 15:31:17,876 INFO [stdout] (default I/O-3) at io.undertow.websockets.core.WebSocketChannel.parseFrame(WebSocketChannel.java:122) ~[undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,876 INFO [stdout] (default I/O-3) at io.undertow.server.protocol.framed.AbstractFramedChannel.receive(AbstractFramedChannel.java:257) ~[undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,876 INFO [stdout] (default I/O-3) at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:20) [undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at io.undertow.websockets.core.AbstractReceiveListener.handleEvent(AbstractReceiveListener.java:15) [undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92) [xnio-api-3.2.0.Final.jar!/:3.2.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:615) [undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at io.undertow.server.protocol.framed.AbstractFramedChannel$FrameReadListener.handleEvent(AbstractFramedChannel.java:601) [undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at org.xnio.ChannelListeners.invokeChannelListener(ChannelListeners.java:92) [xnio-api-3.2.0.Final.jar!/:3.2.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at org.xnio.conduits.ReadReadyHandler$ChannelListenerHandler.readReady(ReadReadyHandler.java:66) [xnio-api-3.2.0.Final.jar!/:3.2.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at org.xnio.nio.NioSocketConduit.handleReady(NioSocketConduit.java:87) [xnio-nio-3.2.0.Final.jar!/:3.2.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at org.xnio.nio.WorkerThread.run(WorkerThread.java:531) [xnio-nio-3.2.0.Final.jar!/:3.2.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) Caused by: io.undertow.websockets.core.WebSocketFrameCorruptedException: UT002015: Extensions not allowed but received rsv of 4
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at io.undertow.websockets.core.protocol.version07.WebSocket07Channel$WebSocketFrameHeader.handle(WebSocket07Channel.java:252) ~[undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) at io.undertow.websockets.core.WebSocketChannel.parseFrame(WebSocketChannel.java:112) ~[undertow-core-1.0.0.Final.jar!/:1.0.0.Final]
2014-04-07 15:31:17,877 INFO [stdout] (default I/O-3) ... 10 common frames omitted