1. A lot of grammatical rules that are technically incorrect. For example, never start a sentence with a conjunction. It is grammatically valid to do this but for some reason primary and secondary schools keep pumping out this myth that it is incorrect and bad writing to start a sentence with a conjunction.
2. Essay writing at secondary school. The way you are taught to write essays at schools is completely useless to you if you are studying a essay-based subject at degree level like English Literature or History. At school, I was repeatedly told that always make sure to show other points of view. So, I was told to say phrases like 'on the other hand, it could be...'
When I got to university, I was told to stop writing this way as using the passive voice undermines my own argument. At university level, I was told to focus more on my own argument as that is what an essay is meant to be. I have never used any of the essay tips I was taught at school in the three years of my degree.