It’s inevitable that if typos get through the checking process, they will turn up in an education policy as happened with National yesterday. Worth noting they were typos rather than actual spelling errors (such as wrong version of principal/principle).
I recall in 2003 I was in charge of producing the education policy discussion document (around a 30 page publication). I was paranoid about even a single mistake in it, and literally checked it around six times myself plus had it reviewed by half a dozen other people around the office. Interestingly each person always finds something that no-one else did.
I actually borrowed from the parliamentary library the NZ style guide for written publications (used to be published by GP Print) and got so anal I was checking whether one should use % or percent or per cent (the latter is preferred), semi-colons vs commas, whether you use ten or 10 etc. We had an external reviewer also and when I commented to her that I had checked it against the NZ style guide, I remember her asking which edition as she then commented she used to be the editor of it. That reassured me that it was going to be pretty bullet-proof and we then had a geeky conversation about obscure grammar conventions.