This happened to me a couple times:
At the end of the Covenant mission, where you activate the light bridge and enter the room where Cortana pops up on your screen and does her dialogue thingy,
If you manage to trigger the cutscene while she is still on your screen, the room fades to black and softlocks your game, only giving you access to the start menu. Reverting to the last checkpoint still kept the black screen, making me restart the mission.