I have had a couple of instances where I find items accidentially hidden behind other items. Is there a way to prohibit space planning from being able to put one item behind another?

Most likely this is occurring from dragging and dropping an item. If you use R, L, U and D to move a position you shouldn't have this problem. If you must drag and drop be sure that the secondary "drop here" arrow points to the side of the position before you let go of your mouse. If your "drop here" arrow appears curved this is indicating that the item will be dropped behind the existing position.

I don't think there is a setting that will prevent it. We use the drag & drop method as well and always make sure users pay attention to the arrow shown. If the hidden item(s) shouldn't be on the planogram, perhaps you could view a positions table to see if items are on there that shouldn't be. Depending on the number and size of your planograms, this would take a while and might not be worth the time.

Depending on the depth of your products and the depth of your fixtures, you might notice some warnings too if the position is outside of the merchandiseable space.