It makes no sense for the escorting fighter planes to fly next to the airliner they believe to be hijacked because that is not a position from which they could take immediate action. They would in fact be taking station to its rear.
In the news report at the end of the movie everything is in English, except for the word "tilkynna", the Icelandic word for "report". Icelandic news does not use that word in news reports and there was no reason for this word being there when everything else was in English.
During the fight in the lavatory, a phone is dropped into the basin which is then filled with water. During the fight, the basin is empty again in some shots, then towards the end of the sequence it is filled with water.
Early in the movie, Jen Summers and Bill Marks order beverages while seated on the plane. The flight attendant brings over a gin and tonic and a water and places them in the cup holders. In the very next scene that shows them in their seats, the drinks are gone.
When the plane suffers the sudden drop in altitude, Marks and at least one of the flight attendants are thrown forcefully to the ceiling of the cabin. Marks, in particular, is then thrown about sideways across the cabin and lands in the opposite aisle from the one where he was originally standing. Moments later, Marks, in a moment of reflection, is shown to be looking at a table top where a variety of liquor bottles and glasses are sitting, upright and apparently undisturbed. Had the plane actually suffered a sudden violent drop in altitude, these obviously unsecured items would have been thrown around and quite likely broken, yet none of the items appeared damaged.
When Bill kills Hammond in the lavatory, he places him on the side of the toilet with Hammond's head bowed down, eyes and mouth closed. When Bill later shows Hammond's corpse to Nancy, his head is now laid back and his eyes and mouth are open.
Up until the point where the fighter jets start escorting the plane the co-pilot Kyle Rice is seen not wearing his jacket, subsequent scenes show him wearing his jacket. It is not explained why the co-pilot thought it necessary to leave his seat to put on his jacket.
The surroundings of the airport in Keflavík, Iceland, is shown as inland, with green fields with some snowy mountains in the background. The actual airport is just by the sea, surrounded by lava and there are no mountains close by.
During pushback from the gate there is no tug attached to the nose gear of the airplane and it looks like it is reversing by itself. Although commercial jet aircraft have the ability to use reverse thrust (the same type of thrust used during landing) and push back under own power if need be, this is hardly ever done.
Bill says that the suspect making the calls will be put into "federal custody" on arriving in London. No law enforcement agencies in the UK are known as "federal", they would be met by the Metropolitan Police and the UK Border Agency. However, it is quite possible that he would request that the airplane return to a U.S. airport.
The fact that the old lady that claims to see Jenn enter the bathroom after her is never explained at the end of the film. Also how could nobody notice the second poison victim's death and how could he not have noticed a phone in his jacket.
Before Bill Marks hands Reilly the pistol his charges the weapon. Later on Reilly discovers the weapon is empty. If the weapon was empty the pistol would have locked in the open position when charged if there was no round in the magazine.
When Bill sends his second message to the unknown assailant, the message sends before he presses the Return key. Also, before the 'floating messages' scenes, there was no auto-correct, but soon after, the word 'supposed' was added using auto-correct.
The goof items below may give away important plot points.
Errors in geography
Assuming the film is set in 2014 when it was released, then the plane would not have been accompanied by Typhoons while in Icelandic airspace. An Italian contingent were there shortly before the film went into production but were replaced by United States F-15s.
During the emergency descent a screen displaying the altimeter is shown. The altitude is decreasing by well over 2000 ft/s. This is well over the speed of sound, and the plane would have been ripped apart long before reaching that kind of speed.
As the plane levels out from the dive down to 8000 ft, the cabin experiences weightlessness. This, however, would only occur if the plane began to dive after a quick ascent, not from quickly pulling up. The people in the cabin would actually be forced down into their seats due to the increased G force.
Federal agents do not go through the same security screenings as civilian passengers as seen in the film. As he was flying armed, Marks would have been required to complete paperwork and would have been taken through a separate screening. (This is alluded to when Marks realizes that the bomb aboard the plane was smuggled via his colleague's briefcase, which was not screened).
The fighters use their afterburners while escorting the passenger plane. Eurofighter Typhoons can easily keep the speed of an airliner without afterburners. Therefore, it's highly unlikely afterburners would be used as they consume extra fuel.
When Bill and group were talking about the bomb, great efforts were made to explain the pressure trigger. In particular they were told that it couldn't be ejected from the plane as the pressure change would detonate the bomb. So when shots were fired and cabin depressurised at 28,000 feet, why did the bomb not explode?