Why won’t it give me the “That Just Happened” achievement?
Well you need to complete The Library without dying on Heroic, if you aren’t getting it, you are dying.
> Why won’t it give me the “That Just Happened” achievement?
If you are dying, you won’t get it. As soon as you see Chief die, no amount of restarting the checkpoint or level will do any good. You have to reset the entire level’s progress and begin again. You can do this by selecting another level and re selecting the Library, change the difficulty etc.
Also, if you split screening with a friend, it’s much harder to get the achievement. If one of you dies, then neither of you will get the achievement when you finish the level.
However, if you are playing with a friend on xbox live and he dies, you still have a chance of getting it so long as you do not die.
Good luck, OP. That Just Happened is a rather frustrating achievement, as it takes both a measure of luck and a rather extensive knowledge of the level’s structure, Flood cues, etc.
i almost had it was runnig for the index right before the cut-scene started got beat down by a covieflood
Have the skull that gives you unlimited ammo on and it’s relatively easy. The only reason I died was because I was using a rocket and an infection form jumped in front of me as I fired 
the only reason I died is because I kept falling down them pesky holes covered by glass.