Your issue is that your animation changes the balls orientation on the y ( yaw ) axis, when you press left you change the x ( pitch) axis. Should this not also be the y axis??
By then using moveenttiy 0,0,.5, you're effectively moving the sphere in the direction that it's z axis is pointing. Sometimes upwards due to the x axis changes when pressing left.
What you could do is create a pivot, parent the sphere to the pivot, animate the sphere but turn the pivot on it's y axis and move the pivot as you're doing.
Stevie
By then using moveenttiy 0,0,.5, you're effectively moving the sphere in the direction that it's z axis is pointing. Sometimes upwards due to the x axis changes when pressing left.
What you could do is create a pivot, parent the sphere to the pivot, animate the sphere but turn the pivot on it's y axis and move the pivot as you're doing.
Stevie