In the 3rd scene at about 8 minutes into the episode, Fonzie, Richie, Potsie and Ralph are back at Arnold's and Fonzie is getting ready to play the new pinball machine (Bally - Space Time) after beating the old machine earlier in the episode (Bally - Nip-It). While he prepares to play the game, Richie suggests that Fonzie show his families garage apartment to his grandmother to get her to move out. When the scene cuts to Fonzie thanking Richie as they get ready to walk out, the pinball machine has reverted back to the old Nip-It machine.
After beating the old pinball machine ('Nip It' by Bally), Fonzie and the gang walk away. The camera follows them, then cuts back to a shot of Fonzie's grandmother standing by the machine. The score display doesn't reset until a new game is started, so the machine should still be displaying Fonzie's previous score, but the counters are all at zero.