In the Infinispan test suite we use a custom TEST_PING protocol to speed up discovery and it sometimes returns Collections.emptyList(). This triggers an UnsupportedOperationException in MERGE2:

2011-10-09 10:34:30,381 ERROR (Timer-5,Infinispan-Cluster,RehashAfterPartitionMergeTest-NodeA-56050) [org.jgroups.util.T
imeScheduler2] failed executing task org.jgroups.protocols.MERGE2$1@4dbce33
java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at java.util.AbstractList.add(AbstractList.java:108)
at org.jgroups.protocols.MERGE2$FindSubgroupsTask.findAllViews(MERGE2.java:325)
at org.jgroups.protocols.MERGE2$FindSubgroupsTask._findAndNotify(MERGE2.java:255)
at org.jgroups.protocols.MERGE2$FindSubgroupsTask.findAndNotify(MERGE2.java:246)
at org.jgroups.protocols.MERGE2$1.run(MERGE2.java:202)
at org.jgroups.util.TimeScheduler2$MyTask.run(TimeScheduler2.java:523)
at org.jgroups.util.TimeScheduler2$Entry.execute(TimeScheduler2.java:428)
at org.jgroups.util.TimeScheduler2$1.run(TimeScheduler2.java:294)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:679)