At the start of the Monopoly game, Woody announces that he's "buying Reading Railroad and ... putting up a house." Ordinarily in Monopoly, a player can't put houses or hotels on the railroads; only on "street" properties, and only if the player owns the entire color group. However, Monopoly is rarely played strictly by its written rules.
When Norm pulls a one dollar bill from Cliff's wallet in the opening scene, it isn't real - it's either a negative of a real dollar bill (the "O" in "ONE" being at the right), or just plain not a real dollar bill. When Carla pulls a $20 bill from his wallet, it appears similarly "backwards", but it's more difficult to evaluate than with the one dollar bill.
The reason the "O" in one is on the right side is because the one dollar bill is being held upside down.
When Frasier rolls a '5' in Monopoly, he actually moves his game piece about 7 spaces on the board rather than the 5 that he should have.