After the big "F-5" comes at the end and Jo and Bill go through the tornado while strapped down to the pipes in the barn, notice that when the tornado passes and the rest of the storm crew arrives, Jo's strap she was tied down with is broken!
I don't think this is a slip-up, it might got broken 'cause she was pull by the "F-5"; if you tell me that afterwards she has the strap on it's place again, now you'll be talking about a slip-up.