Other mistake: When all of the raptors look at Owen from the perspective of their head mounted cameras, they are all looking at him from roughly the same angle, despite being a distance from each other. (01:34:05)
Other mistake: When Chris Pratt is driving his bike along with the raptors the terrain is completely covered with bushes and broken trees and in couple of shots the raptors themselves were seen jumping over it, but he is able to cruise smoothly without any after effects of the bumpy road. (01:31:00)
Other mistake: When the ACU are trying to capture the Indominus Rex, scenes cut back to the control room. The vital signs for the troopers, who are supposedly fighting for their lives, have normal resting rhythms. (00:45:10)
Other mistake: When the I-Rex finds Owen and Clare at the visitor center garage, Owen's gun is leaning up against the JP jeep. The I-Rex knocks up the jeep, but the gun doesn't fall over.
Other mistake: Near the end where the male lead is shooting his lever action rifle, he shoots it as if it's a semi-auto.
Other mistake: When Claire is running to the quad bike she gets a phone call and answers it. However, 1) the phone doesn't light up or show any indication it's ringing and 2) she doesn't press anything but somehow answers the call. (01:19:00)
Answer: Chances are either the gates were left open after the Pterosaur attack to allow visitors to escape quickly, or Hoskins had left them open earlier and forgot to close them to try and justify using the raptors as weapons.