Unacceptable orthography - folk spelling edition
I have argued in the past that kanji should not have eyelashes. I now propose an additional rule: kana should not exhibit anal bleeding.

ぢ means hemorrhoids. The word is Sino-Japanese, and the kanji is 痔, with an official, "dictionary" spelling in kana of じ. Nevertheless, ぢ is far more common. Why?
To backtrack a little bit, what's the difference between じ and ぢ anyway? In terms of pronunciation, nothing: most of the Japanese-speaking community does not distinguish between the two any more. And the standard modern rules of orthography reinforce this by mandating じ instead of ぢ, even when the latter would be more etymologically correct. Same goes for ず instead of づ.*
Now, it turns out that the old, pre-modernization spelling/pronunciation of 痔 was indeed ぢ. So the question is, how did it survive? Answer: with the assistance of hemorrhoid-healing concern Hisaya Daikokudō. (Warning: close-up photography.) They consistently employ the ぢ spelling in advertisements and elsewhere, and since this enables people to disambiguate piles from the many other words that じ might represent, the community embraced the concept. The end result is that ぢ is the the folk-standard spelling, no matter what official policy says.
There is a technical linguistic term for this. It is "sticking it to the Man."
Charles:
There's a vaguely related English-language tale about Bayer Aspirin. They started using a slogan "ADult Asprin," deliberately misplacing the accent on the first syllable, instead of the usual accented second syllable of "adULT." They wanted to see if they could measure penetration of the commercial by seeing if they could shift the accent in common usage. It seemed to work, it's been years since the commercials ran but it is still fairly common to hear people use the wrong accent. This annoys me terribly.