I'm been starting to try to read some Japanese novels recently, and one thing that struck me was that when furigana is used, it seemed to show up in the most unnecessary places.
For example, in the first few pages of Haruki Murakami The Wind-Up Bird Chronicle (ねじまき鳥クロニクル), furigana was given for
猫 【ねこ】、椅子【いす】、眺【なが】める、泥棒【どろぼう】、恰好【かっこう】、悪戯【いたずら】、and 蜘蛛【くも】,
all of which are common words that I'd guess are easily readable by most Japanese 10-year olds.
Maybe the last two are there since those words are usually written in kana, but the first two for example, make no sense at all.