FULL PRODUCT VERSION :
java version "1.6.0_12"
Java(TM) SE Runtime Environment (build 1.6.0_12-b04)
Java HotSpot(TM) Client VM (build 11.2-b01, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Windows XP Professional - Version 2002 - Service Pack 2

A DESCRIPTION OF THE PROBLEM :
Throwable class is not thread safe regarding private StackTraceElement[] stackTrace field.

Can be modified concurrently by two threads causing ArrayIndexOutOfBoundsException.

Two places of modification:

1) setStackTrace
    public void setStackTrace(StackTraceElement[] stackTrace) {
        ....
        this.stackTrace = defensiveCopy;
    }

2) getOurStackTrace
    private synchronized StackTraceElement[] getOurStackTrace() {
        // Initialize stack trace if this is the first call to this method
        if (stackTrace == null) {
            int depth = getStackTraceDepth();
            stackTrace = new StackTraceElement[depth];
            for (int i=0; i < depth; i++)
                stackTrace[i] = getStackTraceElement(i); // throws ArrayIndexOutOfBoundsException
        }
        return stackTrace;
    }

Solution: setStackTrace should be synchronized as well

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run two threads calling on the same Throwable object:
1) first thread - setStackTrace
2) second thread - getStackTrace

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Not exception thrown
ACTUAL -
Occasional ArrayIndexOutOfBoundsException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.ArrayIndexOutOfBoundsException: 65
	at java.lang.Throwable.getOurStackTrace(Throwable.java:592)
	at java.lang.Throwable.printStackTrace(Throwable.java:511)
	at org.apache.log4j.spi.ThrowableInformation.getThrowableStrRep(ThrowableInformation.java:59)
	at org.apache.log4j.spi.LoggingEvent.getThrowableStrRep(LoggingEvent.java:342)
	at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:304)
	at org.apache.log4j.RollingFileAppender.subAppend(RollingFileAppender.java:236)
	at org.apache.log4j.WriterAppender.append(WriterAppender.java:159)
	at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:230)
	at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:65)
	at org.apache.log4j.Category.callAppenders(Category.java:203)
	at org.apache.log4j.Category.forcedLog(Category.java:388)
	at org.apache.log4j.Category.log(Category.java:823)


REPRODUCIBILITY :
This bug can be reproduced occasionally.