Grandpa's Packard / Phoebe's Little Problem - S5-E19
Other mistake: Phoebe's Little Problem: When Arnold goes into Phoebe's room to cheer her up, the birdcage is reflected in the mirror on the wall, but the birdcage is too far to the right of the mirror, so it shouldn't be reflected. This error is shown again in a later shot where Phoebe tells Arnold, "And I'm very good at checkers." (00:09:13 - 00:09:45)