In the part where Spiderman is fighting Doc Octopus on the train, Spiderman's mask gets burnt and he took it off. After Doc Octopus went away and Spiderman stopped the train, he passes out. The passengers stop him from falling and carry him over their heads. When the little kid gives him his mask back its not burnt or scratched at all.
Or, if he took off the mask while the train was moving, and he took it off outside, how did it end up inside the train towards the front where the kids could find it? He took it off near the middle cars, and threw it off the train.
Maybe it was not a burn. What do you say about when it´s not fire, it´s just kind of a smoke... and the stain could be that doesn´t fix in the cloth. Like coal... Imagine if you rubbed coal in a plastic cloth, like Spider Man´s. You would have a stain. But if you rub your hand on it, the stain is gone! Do you get it?
First, there were sparks that hit the mask and briefly heated the eyeholes. He ripped it off, because having a hot mask doesn't help. After a few seconds in the wind, it cooled down. The mask floated to one of the connectors between the cars, where one of the kids found it. Actually, if you look closely, I see significant burn marks, and a little hole.