At the end of the movie when Lennie and George meet up in the middle of the stream they are fairly wet especially with Lennie falling in; when it shows them on shore their clothes show no signs of being in the water at all.
After Lennie and Curley fight, Lennie has no scratches on his right cheek. In the next scene, George cleans out wounds on Lennie's right cheek.
When Lennie is first drinking from the stream his hands and the tip of his hat are in the water; a moment later it shows him from a different angle and his hands and hat tip are out of the water while he drinks.
In the scene when Lenny drinks from the stream, he puts his hands in the water, but when he stands up and talks to George his hands and sleeves are dry.
At 19:27, and again at 19:52, the position of Lenny's hands changes between shots.
At various points in the movie, you can see Candy's clenched fist where there isn't supposed to be a hand.
When George and Lenny first arrive at the ranch, Candy quiets the barking dogs by yelling at them. The two in front whimper and quiet, but the third one behind them is still barking despite no sound being heard.
At approximately 1:38:08 just after Curley begins to ride his horse, a crew member or bystander in a white shirt, black shorts, and white shocks and shoes is clearly visible on the right standing next to a tree in the background.