The slip-up is found in the train scene, when Dr. Octavius throws Spider-man fowrwards, the one which Spiderman has to dodge so as not to hit the grills of an overhead bridge, then Dr. Octavius receives Spider-man behind him.

As Spider-man is thrown forwards, it is impossible for him to land behind Dr. Octavius. This is proven impossible by a simple physics theory.

For instance, the train is traveling at 50km/h. Thus, Spider-man and Dr. Octavius, being on the train will travel at the same speed. As Spider-man is thrown forwards, for instance, at a speed of 20km/h. Thus his ground speed will be (50+20) km/h. So, theoratically, Spiderman will land in front of Dr. Octavius. This is because while Spider-man's speed is 70km/h, Dr. Octavius' speed remains at 50km/h.

In other words, the result should be the same as when for instance, Dr. Octavius throws Spiderman forwards while on the ground (not on train).