In the midst of gcse revision this year, I worked with my son through sample English papers. They make available the papers and the marking scheme used by the examiners, online.
Over three papers we worked through, I found several clear errors. I can't remember them all, but one was a multiple choice in which you had to select a definition of 'correlation' and the correct answer was, apparently, where one thing influences another (actually closer to 'causation', and very different); another question asked us to 'synthesise texts c, d and e' - we looked together at length and couldn't find enough in common to allow synthesis and after we'd had enough, we looked at the mark sheet and it became clear that the question meant something like 'synthesise c, d and e, each in turn - don't try to synthesise all three together'.
I was surprised that such issues had made their way into an exam that could change the future of someone's career. When you're struggling to get the C grade you need to carry on studying in the school, every mark matters!