Welocme to KnittingHelp!
I agree, maybe shaping the toe or heel.
There's a mistake in the pattern. You can't repeat row 13 exactly. I think what you should do is something like row 13, row 14 and after the turn, knit to one stitch before the small gap (from the previous turn), k2tog, k1, turn. You can repeat that, each row decreasing one stitch until you get to 35sts. It's a bit like a heel turn for a sock. http://www.knittinghelp.com/video/pl...heel-of-a-sock
There are errata
online for that issue of Simply Knitting butI don't see the maryjane slippers.