When Maggie and Jordan are talking in the locker room and Jordan ask her to marry him, you can see his mouth move, several times, when the camera is behind him, but he doesn't speak in the scenes with the camera behind him.
When Meg Ryan listens to Dennis Franz's heart with her stethoscope, she places the stethoscope in her ears backwards. (Ear canals angle forward and stethoscopes do likewise - putting them in backwards is painful and can also prevent them from working) Ouch!
When Maggie is in the bath-tub, Seth can clearly be seen in the corner of the mirror when Maggie runs her hand through her hair. Then in the next shot (the famous 'no-reflection' scene), he has no reflection.
When Maggie is riding her bike to the hospital at the beginning of the movie, the seat is clearly adjusted quite high. When she parks the bike, the close-up of the bike shows the seat only about two inches above minimum.
When Maggie sees the truck she is about to hit towards the end of the movie, the truck is coming from the right side of the road and making a right turn. Later on when Nicolas Cage comes around to where Maggie is lying, the truck position is in the opposite direction, like it came from the left side of the road.