Parent Trap, The (Original Version) - Boy, That Popcicle Melted Fast...
OK, this takes place in the scene where the two of them are talking in their cabin while its raining. One of them, who is eating a popcicle, walks to the far end of the cabin. She turns, her back facing the camera. When she turns around again- NO POPCICLE!
This really does happen- it's the part where Susan comes up with the idea to switch places. She starts pacing the cabin- steps up onto a bed (all the time with the popsicle in hand) and as soon as she turns around to say "You want to meet Father, and I'm just DYING to know Mother..." the popsicle's GONE! Too funny!
Given the circumstances happening in the scene (the idea to switch places), I would suggest Susan was so shocked at her idea, she just dropped the popsicle on the floor!
Regardless, it does just disappear from her hands, and it definitely looks like a slip-up!