Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 196-197.
Exodus 34:21 states: "Six days you shall work, but on the seventh day you shall rest; even in plowing time and in harvest time you shall rest."
According to Fishbane, this verse reflects an attempt to render the Sabbath law more comprehensive--to make clear that people shouldn't be harvesting on the Sabbath, even though they may fear that they'll go hungry. This reflects Fishbane's belief that legal exegesis was intended to apply to real life.
Of course, whether people followed that rule is another matter. But Fishbane may be right when he says that the author of this verse was trying to make the commandment more comprehensive, or, more accurately, to clarify that prohibited "work" included agricultural activity.