The worst instance of this was with Minecraft. I didn't realize the achievements unlocked in the demo would carry over.
As soon as I created my first world I got "You unlocked 13 achievements for 170G"
However, there was a game where I got 3 or 4 achievements simultaneously. One for killing the final boss, one for finishing the game on a certain difficulty, one for getting a certain number of kills with a specific weapon.
~I was stomping goombas and saving princesses long before most of you were born.
One of my secondary profiles that never touched the DLC earned like 5 of the Zombie Island of Dr. Ned achievements when I did a split-screen game with it and my primary profile, which had completed the DLC.
I don't know if this is a known bug, a random fluke, or I unintentionally did something that made them pop from just starting a split-screen game.
R.I.P. Eve English (Feb. 12, 1968 - Oct. 13, 2010) Momma Eve, you will be missed.