Harry Potter and the Prisoner of Azkaban - Bed Sheets
In the scene where Harry wakes up in the hospital wing and Hermione tells him that the dementors are going to give Sirius the kiss, you CLEARLY see that Harry is lying ABOVE the bed sheets and the bed is still made. Then, when Harry walks over to Hermione in the centre of the room so that she can put the chain around his neck, you see behind him that the bed is un-made, and it looks like someone just climbed out of it. Then, when Hermione turns back the time-turner, you see Harry going back in time into bed, and he goes UNDER the covers. I think that this slip-up was pretty notice-able.