Continuity mistake: Sue's hairstyle, as well as the texture and length of her hair, and the color, changes from scene to scene, and even at times from shot to shot. This was due to movie re-shoots.
Plot hole: Dr Doom has amazing powers, able to crush people or pop their heads in an instant, which he does to all the humans but not to his biggest threat, the Fantastic 4.
Other mistake: If you look closely after the film leaps ahead a year, Reed's file on Ben lists his birthdate as 1986, but the prologue of the film which features Ben and Reed at roughly 10 years old each is set in 2007. The math doesn't add up.
Continuity mistake: Young Reed has blue/green eyes but older Reed has light brown eyes. Eyes never change color with age.
Continuity mistake: When Franklin sees Johnny on fire for very first time, he is lying on a pad, but in the next shots the pad is gone.
Answer: It is believed that the heat from the radiation caused Reed to start "melting", which is how he got his rubber powers. It is unknown why Sue got invisibility.