When text is highlighted (such as after it is found in a search) and you do a cmd-F for another search, that highlighted text is put in the search box. First, is there a pref for this? But more importantly, that text should be highlighted in the search box, so that if you don't want it, you can simply type to replace it.
I'm constantly hitting cmd-f when I happen to have text highlighted and then typing my search term, only to have it added to what's now in there. This strikes me as highly unusual behavior. I can't think of another app where this happens to me.