Coronation Street - Becky's Disappearing Friend
In the "Becky's Wedding" episode, the stylist suggests the drunken Becky goes with her to the salon to have her eyelashes tinted.
As they walk across the road, Becky spots fiancee Steve approaching, and flattens herself against the wall of the building opposite the cafe. As Becky peers around the corner at Steve, the camera pulls out to reveal the stylist has disappeared in a flash. Becky runs to get away and there is no sight of the stylist in any direction. There is no more sight of her, nor any more mention of her. This was a major, and very obvious Slip-Up!