Lucy Gets a Paris Gown - S5-E20
Visible crew/equipment: Lucy is walking around the living room, assembling a sandwich from food she's hidden in various places. When she tells Ethel that they're low on lettuce, the shadow of the boom mike is visible behind her.
Chosen answer: Keep in mind first that real money has a tendency to disappear. So a production is often not going to risk the loss of X amount of money by using the real thing (this is especially true in bank scenes or "raining money" scenes). So the property (props) department on the production will make fake, worthless bills. But... the fake bills can't replicate real bills too closely or they violate U.S. counterfeiting laws. This was especially true in older days when counterfeiting was much easier. Currently, the laws are less restrictive so it's not as obvious. But there are still guidelines (like color and size of bills) that film and television producers must comply with in order to produce fake money.