The Story of a Divorce - S1-E14
Revealing mistake: In the scene where Reba and Parker kiss outside the restaurant, you can see creases in the scenery backdrop behind Reba.
Up a Treehouse with a Paddle - S1-E21
Revealing mistake: When Brock finds Reba holding baby Henry in the kitchen, you can see a doll's face in the blanket.
Skating Away - S2-E2
Revealing mistake: When Van falls from the roof, you can see that the backdrop gets hit and starts to move.
She's Leaving Home, Bye Bye (1) - S3-E1
Revealing mistake: When Kyra opens the front door to leave, you can see the seam of the backdrop running up through the "grass" across the street.
Answer: I believe it was "Seeing Red" from season 2.
Karoo