Continuity mistake: In the beginning of the movie as Hummel visits his wife's grave, the rain changes from shot to shot, in some it's raining heavily, while in others barely at all. In the shot just before we see who's grave it is you can see blue sky behind Hummel.
Continuity mistake: The SEAL Team attempts to take the island in the cover of night. When Hummel expects invaders, he sends out patrols to look for them. In every shot of the patrol searches it is night. Except for one shot where it is pure daylight.
Continuity mistake: When Goodspeed is driving through the warehouse in his yellow convertible then smashes through the big glass window you'll notice that it's simply not possible. The window is about a foot off the ground, which means there would have to be a ramp. If you look closely there are two hinges at the bottom of the window which show where the ramp ends. The fact that there's a ramp at a dead-end window was poorly planted there by the crew.
Chosen answer: Alcatraz is small enough and out in the middle of the bay that any lookout in a high point could see and hear a chopper, no matter where it approached from. The idea of the decoys was to split any enemy fire received. The whole insertion of SEALS was absurd, in a real situation like that, the SEALS would have been inserted by submarine or used dive motors to pull themselves to the island.
Grumpy Scot