Following the release of OS X 10.10.3 on April 8, Apple has pushed out a small update for that version. The update fixes a video driver issue that could prevent your Mac from starting up. Here are Apple's full notes on this update, which weights in at 1.2MB:

The OS X Yosemite 10.10.3 Supplemental Update fixes a video driver issue that may prevent your Mac from starting up when running certain apps that capture video.

OS X 10.10.3 added the new Photos for Mac application, over 300 new emoji characters, and several other enhancements. Apple also made some enhancements to Mac graphics drivers, making life easier for Mac gamers.

You can grab the update right now from the Updates section of the Mac App Store.