So, every once in awhile, my wife's MBP is doing this. It still functions okay. When she logs in it goes back to normal. And it's not just the log in screen. It randomly happens while she's browsing or using apps as well. But again, if she manages to find refresh or her bookmarks and goes to something else, her screen goes back to normal. I've reset the PRAM and SMC which is basically the extent of my troubleshooting skills. No dice. Any input? Thanks. I believe she's running a late 2011 machine with Yosemite.