You tend to pull the stroke toward your body - especially on the ends of the stroke. Easy to end up with an hour-glass shapped channel also if you don't watch out for this. Just think about it as you are doing it and try to keep the force centered and "down" on the channel (rather than pulling or pushing to one side).

Easy to do if you think about it and alternate sides, but easy to mess it up if you just jump in and start making sawdust!