To reproduce this issue you can use a setup similar to the one used for LBCORE-131, but then use a filename pattern like "log.%d

.%i.txt.gz" for the RollingPolicy.
This works fine as long as you don't restart the application that is doing the logging. The following files are created:

log.2010-08-02.0.txt.gz
log.2010-08-02.1.txt.gz
log.2010-08-02.2.txt     ( = active log file)

However, after a restart the following happens:

log.2010-08-02.0.txt.gz
log.2010-08-02.1.txt.gz
log.2010-08-02.1.txt     (!!)
log.2010-08-02.2.txt.gz
log.2010-08-02.3.txt.gz
log.2010-08-02.4.txt    (the active log file)

So there are now two versions of the logfile with index 1: one zipped and one unzipped. They do not contain the same messages!
Also the order of the messages in these logfiles is not what I expected. After the restart logback will first create the 'log.2010-08-02.1.txt' file and after that it will fill the existing 'log.2010-08-02.2.txt' file to the rollover size.