Knitting in the Round

Last night I met with Arlene to teach her how to purl and how to knit with dpns. She is knitting up a hat for her son. She did great with everything I taught her, it was me who messed things up. I knit the first round of the hat, and she took it from there. When she got to the end of the second round she noticed that there was something wrong. I checked her work and it was fine, it was me who cast on the wrong number of stitches. Don't ask me why I thought the first round went fine, I must have been talking and didn't notice my mistake! Some instructor I am. I stayed up quite late last night reworking the hat and it is now where we left off.