When you work in Photoshop, you can lock layers. This is to prevent anything from happening to that layer, so you are completely prohibited from accessing that layer until it is unlocked. You also can't draw on the layer, and you are then given a warning message.
This is how locked layers should work. However, I've placed objects on locked layers many times by accident, which of course should be impossible. Also, it's possible to place objects from an unlocked layer to a locked layer. This also shouldn't be possible.
