Under the first sense for heterogenous, the OED writes:
Labelling the word as “less correct” suggest a proscriptive approach to usage. Checking the British National Corpus produced 13 instances of heterogenous compared to 278 of the allegedly better form heterogeneous. A similar distribution is found by doing a full-text search of the OED itself: 11 tokens of heterogenous and 206 of heterogeneous. So we clearly have to do with variant spellings here.
The OED revision process started at M, and hasn’t got to H yet, so it’ll be interesting to see what will happen to this word when it is revised.
Any other examples of pre- or proscriptivism in the OED?