For example, after her husband's death in a car accident, Linda (Bullock) travels through time every time she nods off. She'll go to sleep and wake up to find her husband dead, but after a night's rest, she'll awaken to find him alive again. Her initial inability to consult a calendar to discover the nature of her problem is borderline infuriating. Once she realizes that she's inexplicably traveling through time, it gets worse. She'll spend entire days investigating the mysteries of her dilemma, all the while obliviously setting up situations she should know will eventually lead to her husband's death. All of this suggests that she's incapable of altering the timeline. But then we get scenes at the beginning of the film where she places stickers on the glass door of her house to ensure that her children don't accidentally smash into it. Later, her daughter accidentally smashes into a stickerless glass door, suggesting that the timeline was somehow changed. Whether the timeline was changed or not, the scars of this accident don't appear on her daughter's face until several days later, which is not a temporal paradox so much as a screenwriter's error.
The joy of viewing a movie like this comes in watching the temporal puzzle pieces gradually and cleverly come together to form a consistent and perhaps tragic, inescapable timeline. But if some or most of those puzzle pieces are missing or ultimately do not fit, the game becomes an aggravating exercise in futility.