Hi -
Thank you for your report. Hopefully this was a one time occurrence for you, but if it does occur again can you take note of the exact steps you did that caused the issue and list that information here with as much detail as possible including the following information:
Version of Engine Build : 4.7.6
Binary or Compiled (Launcher or GitHub): Launcher
[Steps of Reproduction - include any details you can about the level / editor setup and current working environment that you can remember, working in Matinee is good, but working in a Matinee with a Director Group, 2 cameras and a Particle Effect being controlled via my Level Blueprint is much more helpful]
Finally if the engine does eventually crash and not just freeze, please make sure to submit the crash report, trying to type the above Steps of Reproduction into the Description Area and post back here with your Machine ID # displayed on Crash.
Hopefully as I said this issue will not recur for you, but if it does please post back here with the additional information and we will begin looking into it immediately.
Thank You
Eric Ketchum