Man alive, that's a bad one - but hard to spot as mostly one is mesmerized by the antics of J & E Lucky spurious guy though, he got a closer look & that mule only wanted to join in!
I'll weigh in on the Twist house while I'm here - I don't think it can be categorised as a production mistake. I think it is artistic licence/an exagerration of the starkness, and it's quite possible that they wouldn't have had "mod cons". One of my relatives only had an outdoor privvy until the late 90s in central UK & another one was using a tub & mangle to do the laundry into the 80s. People from earlier generations tended to have more of a frugal lifestyle.
I take it a "mangle" is what folks in the U.K. call what folks in the U.S. generally call a washboard--like when Alma is scrubbing the clothes when she asks Ennis to wipe Alma, Jr.,'s nose?
Tell you what, though, speaking of modern conveniences reminds me of differences in Ennis and Jack's childhood environment as mentioned in the story. In the motel room in the story, Ennis tells Jack how he got his elder brother to stop beating up on him by ambushing him and beating on him instead, and he clearly mentions an outhouse. The Twists, however, had an indoor bathroom--the location of the infamous episode where Jack's father had urinated on him.