After I upgraded Drupal Pathauto module to 5.x-2.0, I kept getting warning: preg_replace() [function.preg-replace]: Unknown modifier 'p' in ..... /modules/pathauto/pathauto.module on line 263.

It took me a while to figure out the solution.

Solution #1:

1. Goto admin/settings/fckeditor/edit/advanced, Add "edit-pathauto-ignore-words" to the exclude field list.
2. Goto admin/settings/pathauto, open "General settings", and remove the added "<p>" and "</p>".

Solution #2:

1. Goto admin/settings/fckeditor/edit/advanced, select "include" in the include/exclude option list, and add "edit-body" and whatever other fields that you want to use fckeditor on.
2. Goto admin/settings/pathauto, open "General settings", and remove the added "<p>" and "</p>".

I think solution #2 is a better way, because this will make sure that fckeditor will not get into the ways of other modules.

More detailed document can be found at http://drupal.org/node/175772, post #7 and #8.