1.Kathy made a serious mistake in the examination.
回答：这句话没有错误.
2.They retired to a small little village in the west of Yorkshire.
回答：在这句话中small和little重复,并且village前面应该用small,所以在这句话中应该去掉little.
3.She realized that it was 12 o'clock.
回答：在这句话中,应根据not until的思想将it was改成until.