I had one newbie who was a space cadet. You give her basic instructions and she would never follow them or understand them.
Go to the register. Ring people up.
See pretty basic, huh? The next thing you know she's in another department chatting away with another person and not where she should be.
Even when the manager gave her instructions, she stared at you with vacant eyes and used the excuse, "I didn't know."
To make a long story short, they let her go.