I'm working on my first scarf now, and somehow it grew from 25 stitches to 32 stitches wide.

Eventually I figured out what I was doing: my yarn is Lion's Brand Wool-Eze, and it can get pretty fuzzy and fluffy. So I'd have between-stitches parts sticking up, and I'd be worried that they were actual stitches I'd dropped!

Easy mistake, but also easy to avoid if you watch where you put the needle.