Depending on the surface...yes. This is why people always say to "cap the corners" of your pen or enclosure. If the surface is something rough like wood they can often scale the corner with their claws. A surface like a smooth plastic would be much less likely to allow a turtle to climb.
Also, they think long term. Mine will move branches and other stuff to a location along the wall she thinks will get her over the wall. They are amazing climbers.