This happened to me just now. I've never seen it before.

java.lang.ClassCastException: comparison between different classes: the other object is null
at org.jgroups.stack.IpAddress.compareTo(IpAddress.java:143)
at org.jgroups.protocols.pbcast.FLUSH.handleStartFlush(FLUSH.java:480)
at org.jgroups.protocols.pbcast.FLUSH.up(FLUSH.java:336)
at org.jgroups.protocols.pbcast.STATE_TRANSFER.up(STATE_TRANSFER.java:144)
at org.jgroups.protocols.FRAG2.up(FRAG2.java:205)
at org.jgroups.protocols.FC.up(FC.java:408)
at org.jgroups.protocols.pbcast.GMS.up(GMS.java:767)
at org.jgroups.protocols.VIEW_SYNC.up(VIEW_SYNC.java:161)
at org.jgroups.protocols.pbcast.STABLE.up(STABLE.java:234)
at org.jgroups.protocols.UNICAST.up(UNICAST.java:267)
at org.jgroups.protocols.pbcast.NAKACK.handleMessage(NAKACK.java:842)
at org.jgroups.protocols.pbcast.NAKACK.up(NAKACK.java:672)
at org.jgroups.protocols.BARRIER.up(BARRIER.java:122)
at org.jgroups.protocols.VERIFY_SUSPECT.up(VERIFY_SUSPECT.java:167)
at org.jgroups.protocols.FD.up(FD.java:322)
at org.jgroups.protocols.FD_SOCK.up(FD_SOCK.java:300)
at org.jgroups.protocols.MERGE2.up(MERGE2.java:145)
at org.jgroups.protocols.Discovery.up(Discovery.java:246)
at org.jgroups.protocols.TP$IncomingPacket.handleMyMessage(TP.java:1535)
at org.jgroups.protocols.TP$IncomingPacket.run(TP.java:1479)