Usually my rMBP wakes from sleep just fine, but now and then the display is black (looks like it isn't even powered up) when I open it, even if I click the trackpad or press the spacebar. This happens even with the battery full and the power cable attached. When this happens, if I close it and open it again a couple more times it finally wakes properly on the third or fourth try. Has anyone else encountered this? Any idea what might be wrong? It's a late 2012 13" model.