We're currently supporting a third party application that, in addition to producing some truly impressive WTFs, generates incredibly verbose log files with zillions of 'error messages' that aren't really errors. This makes diagnosing problems in their server code* very difficult.
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2005/02/gettin-greppy-wit-it.html