Daphne's pin is in a different spot every time she is on-screen.
None of the grenades Nick throws at the bar do any damage.
Nick dives onto the hand grenade, but he is standing when it explodes.
Sergeant Parker's uniform changes from Army green to Air Force blue and back.
When Kevin checks up on Amy and Roadrash in Club Scum, Kevin walks up to the tinsel curtains twice.
During the nightclub chase, the patrons tip over their own tables.
Nick throws grenades in one direction, but the resulting explosions come from a different direction.
Pixie repeatedly swings a bottle at the Club Scum emcee. She never comes close to contact with him, but he cries out in pain.
The same footage is replayed two or three times during the nightclub chase; notice the same guy tipping over his table.
Road Rash decides to fight the monster. When it begins walking, someone's pants pockets are showing through the back of the monster suit.
In McCreedy's flashback, he reads a press kit for the movie The Visitants (1986). His flashback would have taken place in the 1950's or 1960's, a few decades before that movie came out.
A Hobgoblin's squeaker horn squeaks without being squeezed.
When they are driving the car during Kyle's fantasy, a hand visibly rocks the stationary car.
In one hobgoblin attack scene, the arm the hobgoblin puppet is attached to is visible.
Why didn't McCreedy just blow up the hobgoblins in the first place, rather than guarding them for decades?
At Club Scum, Nick addresses Sergeant Parker as "sir" and salutes him. In the U.S. Army, sergeants are not addressed or saluted by junior soldiers in this manner. Those courtesies are reserved for commissioned officers.
Nick is stated to have just completed Army basic training. His hair would not have gotten that long since Basic Graduation, nor would it be in regulations in the Army.