In the scene after the boys meet with Elvis Mitchell, Billy takes his shirt off as he tries to fight E. On his back is a full tattoo that was started a few hours before. The tattoo should have actually taken about 10 hours if done in one sitting. The skin around the tattoo should have been red and inflamed, but it wasn't on Billy's back. His back should have been covered with a bandage as well, to protect the new tattoo.
In the "Official Medellin trailer," there is a mid-90s Lincoln limo, as well as coke being snorted through a current issue $100 bill, despite the movie taking place in 1980.