If you go to school in Japan, which I naturally did (kindergarten thru university), you do not even learn to write おもしろい using kanji. Most teachers would correct you if you learned to write it as 面白い someplace else as in manga or magazines and used it in school. There are many words like this.
Here's a website regardng this phenomenon by a major dictionary publisher.
http://dictionary.sanseido-publ.co.j.../subPage3.html