The Eraser tool won't erase images, text, symbols, graphs, or gradient meshes. The preview, whilst your mouse button is still held down will preview these objects being erased, but when you release the mouse they will actually remain intact.
The Eraser tool will erase any valid object if there is no selection. If you have any objects selected though it will only erase those selected objects. Again, it will preview any objects being erased whilst your mouse button is still held down, even if those objects aren't really being erased. It's a bit of a stupid implementation but hey, what can we do.