Sally and Don's First Kiss - S3-E24
Factual error: When Dick is purchasing his lunch, the cashier tells him the total is $3.75 and he incredulously lays down another coin because he thought his total was going to be $3.50. When he walks away with his tray, you can see that he paid for the meal with 4 coins. Unless he paid with silver dollars (the Sacajawea dollar had not been minted yet) then there is no way he could have covered the $3.75 with just 4 coins. And if he did pay with silver dollars then it would have been completely out of character for him to not have gotten any change, seeing how the show has established him as a cheapskate, as well as he would definitely get change seeing as how upset he was at having to pay an additional 25 cents for his meal.