- A mule is the sterile offspring of a donkey and a horse. A mule and a donkey are two different animals. In the beginning of The Gunslinger (The Gunslinger, Chapter 2) it says that it was a donkey that Roland led through the desert. A bit later, in the same chapter, it says that it was a mule. And the animal stays a mule through the rest of the book.
- In the last chapter in The Gunslinger, Walter tells Roland that atoms are composed of nuclei and revolving protons and electrons. That is not entirely true. The electrons do revolve around the nuclei. But one or more protons make up the nuclei, together with one or more neutrons (except when it is a Hydrogen atom we are talking about. That one does only have a proton and nothing else in it), so a proton can't revolve around itself.
- (Actually, in colloquial English, the word "revolve" can also mean "rotate" or "spin"; for example, consider the Beatles' album Revolver. Protons certainly do spin, though not in a simple fashion.)
- A larger scientific error is in Walter's description of magnifying a pencil tip: "What seems solid to us is actually only a loose net held together by gravity." Most things we consider solid are held together by electromagnetism; gravity is the weakest of the four forces known to physics, and is far too weak to hold together small objects such as pencils.
- On page 71, in The Gunslinger, chapter titled The Way Station, King describes the main character's father as "she" by accident. A quote: "There had been the recurring dream of his room in the castle and of his father, who had sung it to him as he lay solemnly in the tiny bed by the window of many colors. She did not sing it at bedtimes because all small boys born to the High Speech......"
- In The Gunslinger we learn that Cuthbert and Jaime are Roland's friends. But in the other books it is Cuthbert and Alain who are Roland's friends. So, what happened to Jaime?
- Just before the betrayal of Jake, the man in black steps in front of the opening that light is coming through, blocking the light. It says that light is just shining on his sides and through the fork of his crotch. However, it is mentioned several times that he is dressed in a robe. How could the fork of his crotch be visible through a robe?