The instructions refer to a diamond eyelet stitch, but they don't  mention that there are 3 different sizes: over 12, over 8, and over 4.
  There's only one detailed diagram, which is for the over 8 diamond eyelet. The diamond eyelets in the border should be worked over 12; the ones in the centers of the medallions should be worked over 8. The instructions  do correctly say that the ones above and below the second serpentine row should be worked over 4.

  There is no diagram for the "anchored upright cross stitch" (called the "diamond smyrna stitch" in TWSC).

TWSC:

  The diagram for the "very small diamond eyelet stitch" is wrong. It shows (I believe) a diamond Algerian eye over 8 threads. It should be the diamond eyelet over 4.

  The diagram for the "diamond smyrna stitch" (AKA the "anchored upright cross stitch") shows it over 6 threads, but it should be worked over 4 in the Harvest Sampler. (Maybe 6 threads is correct for one of the other charts in the book.)

That's all so far -- these charts are not too bad as far as errors go, but I could see an inexperienced stitcher having a problem.