Note: Don't use any of these in production!
libapache-mod-cgi-debug - just enable and add a hook to the form that is making apache biff - gives nice error page after post!
mod_whatkilledus - complie, and enable (disable after use or you will suffer performance hit - logs to error.log for apache
mod_backtrace - complie, and enable - disable after use or you will suffer performance hit - logs to error.log for apache