mod_log_config with buffered logging turned on shows corrupted log lines when serving at very high volumes using the worker mpm. This is very consistent behaviour with all recent versions of 2.0. The problem appears to be that the per-child buffer management is not thread safe. There is nothing to prevent the memcpy operations in ap_buffered_log_writer by different threads from overlapping. Since this is a problem that happens at high volume, adding a mutex around this isn't necessarily the most attractive option. Also, it is unclear whether or not a thread mutex is good enough. mod_rewrite uses a global mutex for its logging.
A thread mutex should be good enough for handling the buffer and possibly flushing. Each process would be atomically appending a set of complete trace records to the file, and that doesn't have to be handled explicitly by mod_log_config. With buffered logs there will be a much higher incidence of out of order records, but that can happen already. I wonder if the big picture is that for a threaded MPM config you're better off shoving everything to a piped logger to handle in its simple way rather than adding serious mutex contention to the web server. Still, the code to make it functionally correct is relatively simple so it should be implemented and the user should get to decide which is more appropriate.
fix just committed to Apache 2.1-dev... I'll propose it for backport once folks have had a chance to look at it Thanks for your report, and thanks for using Apache!