I'm not 100% sure this is a bug or a 'feature'.. If you do a search on "Chitty on contracts" for example, every instance of 'Chitty' and 'on' is highlighted including the 'on' contained within 'contracts'. Surely the hole word 'contracts' should be highlighted and every occurrence of 'on' inside a word should not..? I've found that if you change the order of the search words to 'chitty contracts on' then you'll get 'contracts' highlighted correctly. but you will also still get 'on' within other words highlighted.
*** Bug 11526 has been marked as a duplicate of this bug. ***