During the store break scene candy girl is run over by a van and passes under the van. Her boots fly off her body with the laces dangling as they fly away from her body. When the driver steps out of the vehicle to shoot her, her boots are back on with the laces still tied.
When Charlie & Leo leave the house, Leo takes down a few guys who are then unconscious.
After that Leo gets shot in the shoulder by the sniper who is then blown up.
One of the unconscious guys later wakes up and tells the mercenaries Leo was hit in the shoulder.
He never could have know.
At the 1:03:10 mark Senator Roan puts the gun in the back of her pants. At the 1:03:11 mark the gun is in her hand at her side again and at the 1:03:12 mark she puts the gun in the back of her pants again.
While the Senator is escaping the underground triage area, someone says to go to 3rd and Thompson. There is no Thompson Street in DC, and all addresses in DC also have a quadrant indicator, NE, NW, SE, or SW.
In the beginning of the movie, we see Senator Roan and her family during the purge 18 years before the events of the movie. However, in the previous movie Purge:Anarchy, the purge has only been happening for 9 years. Since this movie is only 2 years after Purge:Anarchy, there is no way Senator Roan's family was killed in a purge 18 years ago.
In the first Purge film, the announcement on TV commencing of the Purge, states that government officials above Class 10 are immune from the Purge Night, in this sequel the announcement clearly states "and for the first times since its inception, no one has been granted special immunity from the Purge", but the "weapons of class 4 and lower" restriction still applies: although it can't be heard directly from the announcer's voice, the rule appears on the screen during the broadcast (seen in the senator's room).
During the opening scene, a phone is hooked into a speaker playing music. However, the icon displays the "play" button instead of the "pause" button like many devices - indicating that no music should actually be playing.
Immediately after the attack helicopter assaults the triage van we are informed that Rondo (the "good kid" who got shot twice by a rival gang member) is dead. However, a few seconds later, exactly at 56:36 you can see his eyes looking slightly to the left.
The goof items below may give away important plot points.
In the scene when the two angry girls return with the two cars wrapped in Christmas lights, after they are hit by the the medical van you clearly see her boots fly off. But after in the next scene that shows the girls on the ground they both have their boots on all over again.