When Ellie begins climbing the barrier in the "pet sematary", she accidentally kicks what appears to be a hornets nest, and is stung on the leg. Judd appears and immediately removes the stinger, and applies a dab of mud as first aid. Hornets do not leave their stingers behind when they sting. Only bees do this.
For the Halloween scenes, the outside foliage is seen clearly in full green, spring bloom, this would not be the case for late October (Autumn) in Maine/New England.
When we see the shot of the truck barreling towards Elle and Gage, the camera truck can be seen reflected off the truck's steel bumper.
During Louis's first trip to the burial ground, some lightning is seen near the horizon. Thunder follows about a second later. The lightning appears miles distant, and the thunder would have taken much longer to reach Louis's location.