I painfully found a bug within Log4J while working on a project. It appears that intermittantly, the following error will be thrown while using version 1.2.13:
NullPointerException at org.apache.log4j.NDC.get(NDC.java:209)
There is no work around. The best bet (and my choice of action) is to download and use the repaired version 1.3-alpha 8...even though it is still in test mode.