This has been observed once in jdk 9 dev nightly. It is a new test from JDK- 8087112: HTTP API and HTTP/1.1 implementation.
http://hg.openjdk.java.net/jdk9/dev/jdk/rev/e0da6c2a5c32

Error message:
----------System.err:(16/875)----------
java.util.ConcurrentModificationException
at java.util.LinkedList$ListItr.checkForComodification(LinkedList.java:970)
at java.util.LinkedList$ListItr.next(LinkedList.java:892)
at Server.activity(Server.java:54)
at SplitResponse.main(SplitResponse.java:82)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:520)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:92)
at java.lang.Thread.run(Thread.java:804)