I was sparring with one our blue belts. We had been doing some deep half guard stuff in the previous classes. He was behind my open guard and I decided to get the deep half guard. So I go for the legs, trap on between my legs, pull his other leg to my head with my arm. Then start to move to the deep half guard.
And suddenly he just steps past my legs and takes the side mount.
Turns out I forgot to control his leg while moving my hips to get the deep half guard.