On page 741, chapter 31 it says that Harry saw "horseless carriages" but he should have seen the thestrals now that he had seen Cedric dying, as thestrals are visible to those who have seen death.

In Ch. 29 when the IOI eargear is first put on Wade, it is "clamped onto his right earlobe, piercing it in two locations." But then, in Ch. 31, Wade disengages the eargear and describes the pain as it "pulled free of the cartilage on my left ear." So there are two mistakes here: first, the eargear changes location from his right ear to his left, and second, it goes from being in his earlobe (which lacks cartilage) to a part of the ear that does contain cartilage.

While being sorted into houses, Harry looks up at the Sorting Stool, and there are only three people left to be sorted. Professor McGonagall then calls out the names of four more kids.

In Chapter 28, "Homecoming", Fitz falls asleep while drinking wine in his bath. When Chade enters and startles him awake, Fitz is still holding the neck of the wine bottle, which Chade catches before Fitz upends it. This means his hand with the bottle was outside of the tub. But only a page later, Fitz mentions that both his hands are wrinkled from being immersed in water for so long.

In Mammoth Hunters, after Rydag died and Ayla was being the mog-ur, she gave Rydag "her totem" the cave lion, but in Plains of Passage she stated that his totem was the grey wolf.