I had an odd crash with Safari: a web page on slate.com caused Yosemite to freeze and display a solid blank gray screen, like the startup screen but without the Apple logo. I had to turn the entire system off and reboot, where it happened again and again when I opened Safari. Once it even went into a strobe mode, with blinking bars that caused some lingering pulsation in the screen. Eventually I was able to start Safari without opening that bad web page, and all is fine now. (Firefox could view the page without difficulty.) This was particularly vexing because it froze the whole machine (2011 iMac) and kept reopening and freezing if I opened Safari again. All is well now, and there were probably more elegant ways to isolate and fix the problem, but is bugged me for half an hour. I haven't seen a similar problem reported, although I may have just missed it, so I thought to mention this in case it happens to anyone else.