When Huw comes home after being beaten by his teacher, there is no blood or marks on his back.
At Angharad's wedding, from inside the church we see Huw waiting outside, wearing a cap. When they cut to the reverse shot outside the church, the cap is gone.
After the whistle blows about another mine accident, the mother runs out with her daughter-in-law Bronwyn. When she starts to run, and in the first shot at the mine, she has no scarf, neither on her head nor around her shoulders. By the second shot, she suddenly has a black scarf on her shoulders, which she later puts on her head for the iconic line about seeing her husband and her late son Ivor in heaven with God. But this scarf just materialized out of nowhere.
As Mr. and Mrs. Morgan argue about whether Huw should fight, in the camera angle that shows all three characters, Huw is right in front of Mrs. Morgan, but when there's a direct shot of Mrs. Morgan, Huw isn't close to her.
In the chapel right before the woman is shunned, Mr. Gruffydd closes the Bible. The pages are clearly blank.
As the doctor is about to leave the Morgan house, about 30 minutes in, Donald Crisp mouths a couple of words that the doctor then says.
Although a good effort is putting into making the locations look Welsh, the Southern California mountains sometimes seen in the background are too high for South Wales.
The wage reduction proclamation contains the word "labor" (American spelling) rather than "labour" as any British Islander would spell it.