The back streets
Moji no ura-dōri (freely, "The Back Streets of Orthography") has an interesting post about a library poster with インターネット利用 (internet riyō, "internet use") furiganafied as インターネットをつかう (internet o tsukau, "use the internet"). In other words, these furigana aren't just a pronunciation guide -- they're a gloss, giving the native Japanese words roughly equivalent to the Sino-Japanese word below them.
That in itself isn't so unusual. It was all but normal back in the Meiji period, when you could apparently use whatever the hell kanji you wanted to write a word, as long as the furigana were clear. The unsettling thing for me is that these furigana change the very structure of what they're attached to. インターネット利用 is a (compound) noun, but インターネットをつかう is a sentence (or at least a clause). The furigana undermine the kanji at the most fundamental level, but the overall meaning of the poster remains unchanged.
Incidentally, this is all probably for the benefit of very young library users, who might not know the word riyō no matter how it's written.
amida:
With a language in which "adjectives" have renyoukei and "adverbs" have rentaikei, you can't be too particular about parts of speech.