Continuity mistake: When Polka-Dot Man hurts Starro's leg, its minions cry out in pain as well in unison, but they do it holding their knee or thigh in one shot, their ankle/calf in the next. (01:53:00)
Revealing mistake: When Bloodsport rescues Ratcatcher 2 from on top of the car, they don't actually dodge the arm coming for them but are going straight through like it's not even there. (01:54:16)
Continuity mistake: When Harley and not-Milton join the others at the end of the movie, shadow areas appear and disappear between shots (rather obvious when King Shark asks his first "Num-num?"). (02:01:10)
Visible crew/equipment: In the club scene when Polka Dot Man is dancing, when the camera spins around you can briefly see the top of a camera/gimbal.
Other mistake: Bloodsport and the others see the explosions Harley is running from, which are less than a couple miles off. The beach itself should already be lit by the explosions and burning wreckage, which the team couldn't fail noticing as they were approaching. To make the distances more ludicrous, when we see Flag's signal in the control room it's said he's "1 mile north" of them, He's already halfway towards the western isthmus. No reason why it'd take them a whole day to make it to their destination.
Answer: It's a combination of various reasons. Will Smith wasn't available to reprise his Deadshot role, so there was a shift in the story. King Shark was intended to be in the first film, but the CGI was too extensive, so they went with Killer Croc. In this film, they took the opportunity to bring in King Shark as the "monster" character. But, this film isn't really a sequel or reboot. Basically, this is a new storyline with new characters, and many of the previous characters are just not in the film, with no exposition on what happened to them, and they may not even be part of this film's "universe." The fact that some of the actors play the same character in both films only serves to muddle the concept of this film not really being a part of the previous film's continuity.
Bishop73