Continuity mistake: In one sequence (in which Ryback tries to save the drowning sailors), Ryback is running with two machine guns and kills a bunch of bad guys, including one guy with a moustache and cap who falls down a flight of stairs. After the machine guns are empty, he pulls out a pistol, slowly goes into another room and, surprise! the same guy appears, gets shot and falls down the stairs! Its obvious they ran out of guys to kill in the film, so they just ran the same film twice to look more violent.
Plot hole: Why was an F-18 even sent to look for the helicopter anyway? Krill was the only one on the ship at the time that knew who was really on the chopper, the crew only knew about Jordan Tate, and the captain believes it's supposed to be a surprise for him so he's not calling anyone, so who sent the F-18?
Factual error: Why did they send only one F/A-18 to look for the missing helicopter? There would usually be two - a lead and his wingman. I would have expected a Search-and-Rescue type or maybe even the U.S. Coast Guard.
Continuity mistake: When the man tries to attack the terrorist while being moved to their "prison", Strannix shoots the man next to the one who attacks, saying something like "If you try anything we will shoot you and the man beside you". In this scene he is wearing a black sweater. In the scene before and after he is wearing the pink T-shirt and leather jacket.
Continuity mistake: The big black terrorist who always accompanies Stranix loses his grey waiter jacket when he and the other terrorists are herding the Missouri crew into the makeshift prison. You can see this clearly when he shoots a marine prisoner who tries to overpower him. Later, when he follows Stranix into the captain's office, he is wearing his waiter jacket again.
Revealing mistake: When Ryback jury-rigs a timed explosive to destroy the helicopter, he uses a tin of paint thinner which is expected to fall off a hand grenade, thus releasing the spring on the grenade. However, when the explosion occurs you can see over Ryback's shoulder that the paint thinner tin hasn't moved at all, and that the explosion occurs inside the helicopter, not where the grenade was placed.
Continuity mistake: Near the end of the movie, when Stranix launches the two tomahawk missiles toward Hawaii, look closely at where the missiles are launching. they're firing out of the space between the two ABLs (Armored Box Launchers) instead of from inside the launchers.
Continuity mistake: When Seal team 5 is sent in to retake the Missouri, Stranex's man sees them trying to sneak in under the ship's radar. As the team readies to drop in on the ship's bow, look through the windshield of the helicopter. No wonder the Missouri picked the helicopter up on radar. They are way up in the air and well within radar range.
Factual error: As Strannix and Krill walk down the "Broadway" passageway, they pass through several watertight doors. These doors never extend all the way to the deck, so anyone passing through one would need to exert some small effort to pick their feet up to step over the threshold. There is no evidence of this, and they continue walking through as if the deck were uninterrupted.
Revealing mistake: At the part where Ryback places a grenade on the door and the man gets blown away by opening it, the shot just before reveals that it's an obvious dummy (best seen in slow-motion).
Visible crew/equipment: After the lady in the cake bit they leave the room and close the door behind them. When the door closes there is a shadow of the cameraman on it.
Continuity mistake: Steven Segal argues with his superior and gets jailed in a meat freezer. When bad guys come to open it and kill him, they throw the door's lock pin at least 7 feet away (you can hear it slide). But Segal overcomes them, shuts them in, and he retrieves the lock pin which is now suddenly directly below the door.
Factual error: In the last scene, where Chief Ryback is rendering honors to his deceased captain, he is in dress blues, with a gold crow. Since the movie takes place some 2 years after Panama, and Ryback received either non-judicial punishment or a Court Martial at that time for striking an officer, he would have lost his "good conduct" rating, and the crow would have been red.
Continuity mistake: When Steven Seagal is swimming alongside the submarine to place the explosive, the bad guys attempt to hook him. In one shot you see that they were successful with the piece of his wet suit caught in the hook and in a different shot you see his wet suit has been ripped above his right shoulder. As Seagal is climbing up to get out of the water, there is no damage to his wet suit.
Other mistake: In each film Segal plays a chief petty officer, who happens to be an ex-Navy SEAL. In the posters for Under Siege, he's shown in the dress white uniform of an admiral. In the second film he's still depicted as an officer, but he has been demoted to a lieutenant. I guess the producers just thought the officer's uniform looks prettier, even though they're the very same uniform, except a chief has anchors on his collar and no shoulder boards.