A couple times now when my Macbook Pro running Yosemite was left unplugged overnight (with the screen open) I have come back to it like this (That's the drained battery symbol that indicates you need to plug in your Mac) The graphics continue to be garbled like that when it starts up after some charging. The problem is resolved when the computer restarts.
My question is: should I be concerned by this? I have never seen this on other Apple devices. My device is still under warranty, so I could take it in, but if it's not really a problem / won't get worse than this, maybe not worth the effort.