Here's a swatch of Mock Fisherman's Rib.
And for comparison, here is my Mistake Rib swatch on the same background:
I thought at first that Mock Fisherman's Rib was just another name for Mistake Rib, until I read the instructions. Both are worked over a multiple of 4 stitches, plus 1, but Mistake Rib is K2, P2 on every row. Mock Fisherman's Rib is K3, P1 on every row. Who would have thought that two such different sequences would give such similar results?
Of course, if you look closely, the differences show up. In Mock Fisherman' s Rib, the purl bumps either side of the knit ribs are in the same row, and in Mistake Rib they are in alternate rows. The charts below make the differences very clear, but also show the knit ribs and purl ribs which are the strongest feature of both. (As before, a blank square means knit on odd rows, purl on even rows; ● means purl on odd rows, knit on even rows.)
But in the large, the two stitch patterns are pretty much interchangeable, it seems to me. On the other hand, worked over a multiple of 4 stitches, K3, P1 would look nothing at all like K2, P2. Knitting is fascinating!