It gets a little too "keen" on adding stuff to the saved search list too I find. Quite often I find that multiple of those precious 15 slots have been "used up" by the original text that got pasted into the Find box - e.g. with the original chapter number/style names before I got to the next step of replacing with my regex characters.
I suspect it is to do with using ctrl+F in an attempt to "load" the search box with a new selection. Which only works intermittently btw - you can select a multiline piece of text in code view, hit ctrl+F and cross your fingers that the text you selected appears in the Find box for editing, rather than it trying to do a find on just a portion of it like an opening <p> tag.