If you installed the Yosemite 10.10.3 update last week, you have an update waiting. Much like Captain Picard, Yosemite now has a supplemental. According to the description, the update “fixes a video driver issue that may prevent your Mac from starting up when running certain apps that capture video.”
Apple has posted an article about an issue with an app called ManyCam, a webcam effects software package. If any other software is causing the same issue, it hasn’t been called by name yet.
There are also some reports of the latest update of Mac OS Yosemite causing startup times to be much longer, as well as reports of some machines experiencing kernel panics upon login. If you have a new 5K iMac, be careful with all those big beautiful images: Apparently working with large JPG files is also causing some Finder issues, including spontaneous restarting or sometimes throwing a kernel panic first.