Other mistake: Jack takes the moonshine to Floyd Banner and sells 200 gallons @ $5 a gallon, ie. $1,000. When he gives the money to Forrest he tells him it's $2,000 minus his commission.
Other mistake: When Calvin sits down to eat his white cake, the cake is a new slice. When the camera switches and comes back, it is almost gone. When the scene switches again, he is eating a full piece of cake again.