Open the provided blend file. It has two scenes, one red and one blue. The red scene has a cube, the blue scene has composite nodes. In the compositor, I have two render layer nodes, one pointing to each scene. Now, render (F12). Works fine. Now, instead, click the 'render this layer' button on the red scene. Nothing appears to happen. However, switch to the red scene (using the scene buttons), and you'll see that it's rendering. The expected behavior would be that it would show a render progress bar in the blue scene as well as in the red scene, but it doesn't. Furthermore, when the red scene finishes rendering, it doesn't update the blue scene, and so the compositor nevers gets that render layer 'back' from the red scene, so re-rendering just that layer is useless in the first place.

Also, I couldn't reproduce it here, but in one of my other projects, when I tried to render one scene from another using the 'render this layer' button, it caused the entire render to be done, not just that layer (in another scene).

Very confusing stuff design-wise if you think about it. This is probably for the todo.