When Homer poses for his third safety sign picture, the one that reads "sign ahead", the sign's shape changes from a diamond to a square.
In the park, Marge's necklace changes from white to red.
As Sheri and Teri are walking along talking to Bart, one of the twins' bodies disappears and her head seems to be floating in mid-air.
When Marge and the kids catch up with Homer on the bridge just before he jumps, she says, "Gasp! Oh, Homer, how could you think of killing yourself? We love you!" but the movement of her mouth suggests she is saying something entirely different.
Mrs. Krabappel tells Bart that the only available seat is next to Wendell, but when walking to the seat, Bart passes an empty one.
When the children are entering the power plant, Sherri and Terri are not walking, but sliding with their feet not moving.