Hi, I have a situation where the same line of code in some cases produces a stack trace and in other cases does not. I have not been able to reproduce this in a unit test. Has anyone ever seen this? I'm using this pattern: <pattern>%date{yyyy-MM-dd HH:mm:ss.SSS} %-6camlevel [%thread] %class.%method:%line - %message %mdc%n</pattern> Here's the offending line of code - 581: logger.warn( MessageFormatter.format("Rejecting {} due to error: ", object) .getMessage(), exc); "object" is an Object and "exc" is an Exception Example w/o stack trace: 2012-06-06 04:10:25.616 WARN [DiameterEnfManager8] msc.rc.diameter.DiameterEnfManager.handleObject:581 - Rejecting CREATE SessionId=seagull_gx;atcaa;b;c;d Subscriber=[IMSI:xxxxxxxx, IP:yyyyyyyyyy] due to error: java.lang.NullPointerException: null Example w/ stack trace: 2012-06-06 04:14:18.727 WARN [DiameterEnfManager2] msc.rc.diameter.DiameterEnfManager.handleObject:581 - Rejecting REAUTHACK SessionId=seagull_gxx;atca Subscriber=[IP:aaaaa, IMSI:zzzzzzzzzzzzzzzzz] due to error: java.util.ConcurrentModificationException: null at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819) ~[na:1.7.0_02] at java.util.ArrayList$Itr.next(ArrayList.java:791) ~[na:1.7.0_02] at msc.rc.model.ObjectFactory.getSessionAltKeys(ObjectFactory.java:4024) ~[msc.jar:na] at msc.rc.diameter.model.DiameterObjectFactory.saveSession(DiameterObjectFactor y.java:752) ~[msc.jar:na] at msc.rc.diameter.DiameterManager.saveSession(DiameterManager.java:595) ~[msc.jar:na] at msc.rc.diameter.DiameterManager.handleAck(DiameterManager.java:732) ~[msc.jar:na] at msc.rc.diameter.DiameterEnfManager.handleAck(DiameterEnfManager.java:3854) ~[msc.jar:na] at msc.rc.diameter.DiameterEnfManager.handleObject(DiameterEnfManager.java:527) ~[msc.jar:na] at msc.rc.framework.HandlerThreadTracker.handleObject(HandlerThreadTracker.java :51) [msc.jar:na] at msc.rc.framework.QueueHandlerThread.run(QueueHandlerThread.java:103) [msc.jar:na] at abc.util.thread.ManagedThread$ExceptionHandler.run(ManagedThread.java:230) [camiant-platform.jar:na] at java.lang.Thread.run(Thread.java:722) [na:1.7.0_02] Thanks, Dave