Factual error: Take a look at Google Earth - the section of the Golden Gate Bridge Magneto breaks off (tower to tower, with a bit of overhang, 0.9 miles at most, more like 0.85) is not long enough to reach Alcatraz - the two closest points are 0.96 miles apart, and that's the very end of a pier, which most likely couldn't support the weight of the bridge anyway.
Factual error: After the troops are mobilized to fight against the Brotherhood and guard the cure clinics, they make a big show of them turning in their M16s and other metallic items and getting their plastic weapons so Magneto can't use the metal against them. But they wouldn't have had their M16s to begin with; you pick up your weapon after you arrive for mobilization at your unit. They would have showed up unarmed and then had the plastic weapons issued to them right after they reported for duty. (Most likely the scene was made just to get the point across to the audience that they need to get rid of all metal and to look flashy). Prior to the President responding to Magneto's declaration of war by mobilizing the troops, the cure clinics were guarded by civilian law enforcement; not US troops.