Factual error: Roland comes from the mountains in the east, arrives at the Great Western Sea, and then decides to travel north (Chapter 1, section 3). This means that as he heads north, the eastern mountains should be on his right and the western sea should be on his left. But as he walks, the sea is on his right and the mountains are on his left (Chapter 1, section 4).
Continuity mistake: Near the beginning of Ch. 2, it says Eddie Dean is 21 years old. But near the beginning of Ch. 3, he tells Odetta Holmes he is 23. And only about a week has passed between the two events (as Eddie says on the first page of the unnumbered chapter called 'Shuffle').
Continuity mistake: In chapter 6, when Roland first looks through Eddie's eyes in the plane, it is insinuated, upon seeing the stewardesses' uniforms, that he had never seen a woman wearing pants before. However, in the previous book, The Gunslinger, several women in the village of Tull are described wearing slacks - so while uncommon, it would not be a point of marvel to Roland.
Factual error: During Odetta's past, it refers to the struggle for equality starting with the first boatload of slaves and says it happened in Georgia. Slavery started in the English colonies in 1619 in Virginia, and Georgia was the last colony founded, slavery had already existed in the colonies for over a century before Georgia even existed.