When Elizabeth puts Ben's dinner back in the pan, his plate has remnants stuck to it. During the rest of the scene the plate is completely clean.
Although John's new telephone looks like an authentic phone from the 1930s, its' ringtone is that of a Western Electric Model 500, which wasn't issued until 1950.
Grandpa figures out that John and Olivia must be celebrating their anniversary out on Waltons Mountain, which is a huge area, yet it's never explained how he pinpointed their exact location.
During dinner, Jason convinces Elizabeth that John Curtis is crying so he can try to steal her plate. There was plenty of food still in the pan, he could have served himself some after Elizabeth went upstairs.
When David and Mary Ellen are in the break room, a boom mic shadow is clearly visible on the wall behind them.