When in any of the tabs in the Creative inventory except for the Survival Inventory tab, leaves seem to have all of their sides rendered instead of just the three shown when they're in the hotbar or the Survival inventory. It also happens to leaves in any container GUI.

I seem to remember first noticing this in an earlier snapshot, probably 13w36a or 13w37a, but know for sure it was in 13w39b.

EDIT: Please mark as fixed in 1.7 pre-release.