Currently, when this happens with AS, I see:

2013-02-18 16:24:43,837|WARN |[com.arjuna.ats.jta]|(ThreadId: Transaction Reaper Worker 221)|ARJUNA016029: SynchronizationImple.afterCompletion - failed for org.jboss.as.jpa.transaction.TransactionUtil$SessionSynchronization@634ef5a7 with exception: java.lang.NullPointerException

From a related email conversation:

Here's our Logger code:
@Message(id = 16029, value = "SynchronizationImple.afterCompletion - failed for

Unknown macro: {0}

with exception", format = MESSAGE_FORMAT)
@LogMessage(level = WARN)
public void warn_resources_arjunacore_SynchronizationImple(String arg0, @Cause() Throwable arg1);

Here is where we call our logger:
jtaLogger.i18NLogger.warn_resources_arjunacore_SynchronizationImple(_theSynch.toString(), e);

Maybe the message should have the

Unknown macro: {1}

in it, i.e. it change it like so:
"SynchronizationImple.afterCompletion - failed for

with exception

Unknown macro: {1}

"