Continuity mistake: There are seven Wharvey Gals. Three on the stage singing, three with the wife, and one that she's holding. One of the girls even says that there are seven of them. Yet in the end, when Everett and his wife are walking down the street, there are only six. One that she is carrying, and five following.
Continuity mistake: When they first meet George Nelson, the left back seat window is half way open. A few seconds after they get into his car, Delmar gives the machine gun to Nelson through the left back seat's completely open window (the camera shows the guys all the time so we can see that Delmar didn't lower the window when he got in the car). (00:32:25 - 00:33:40)
Continuity mistake: In the scene where Everett finds his wife in the Woolworth's, he pulls her off to the side to have a private conversation. You can see the twine that connects her and all her children start to stretch out as she walks away. In the next shot, as she is walking over to Everett, the string is gone. (01:03:00)
Continuity mistake: As John Goodman is breaking the branch from the tree during the picnic scene, the branch is quite sharply pointed on both ends. In the scene immediately following, when he starts to beat the men with it, it's blunt at both ends, and a slightly different shape. (00:53:00)
Continuity mistake: In the scene near the beginning where the three main characters are running across the field to catch the train, in the shot where the train is in the foreground you clearly see the engine and the front carriage pass by the convicts while they're still quite a long way from the train. In the next shot though which is taken from behind the men with the train in the background they reach the train pretty much as soon as the engine passes them by. (00:04:45)
Continuity mistake: When the trio and George Nelson arrive outside the Bank in Nelson's car it appears covered in dust. The door Pete gets out of is clean and shiny. (00:33:10)
Continuity mistake: In the scene where the Soggy Bottom Boys are singing "Man of Constant Sorrow" at the country radio station, the clock behind George Clooney's head shows 2:15, then 2:25, and then back to 2:10 during the course of the very short song.
Continuity mistake: When Clooney and the 2 other guys enter the political meeting, there's a close shot of Clooney where he addresses Hunter. Meanwhile, the music is playing and the others are singing. Behind Clooney you can see, out of focus, a man playing a guitar along with the music. When the music stops, we go back to Clooney's close shot (where he is still talking to Hunter) but the man behind him is still playing the guitar. (01:24:10)
Answer: Probably they have dismantled the coffer dam that was used while constructing the main dam. Hence, the flooding.
Good and reasonable last resort explanation - have the Coens ever addressed it?