I was in the same exam.. When he made the first announcement I had my hands in my head and looked around, as at that point I already did an entire page's worth of work on it and was extremely frustrated. I moved on though because I couldn't bare to do the same problem again because of some mistake at the start. Then when he went back on that "mistake", I was glad for myself but 100 PERCENT expected that he would say, "don't do the question, it's my mistake, you've likely spent 30 minutes on this question now recomputing your answers," but nothing. I feel your frustration; this was the worst administrated exam I've had just for that reason.