I know this is a very old post, but I encountered the same problem. My pattern called for p2 yb sl1 k2 sl1 yf k2. This creates 4 extra stitches. I was working on the RS. I could only conclude some steps were missing, and changed it as follows:
P2 yf sl yb k2 yf sl1 yb k2. This way I was wrapping the sl st rather than creating a yo. Hope this helps someone else!