(at around 47 mins) When the team see the short recording of Doom's encounter with the Silver Surfer, Sue stands up twice.
(at around 39 mins) As Von Doom lands his helicopter on the slab of ice to meet the Silver Surfer, the helicopter points toward the left side of the screen, but a few moments later, after the cut scene to the hole, the helicopter is seen pointing towards the right side of the screen. Another cut to the hole and back, the copter is back to facing the left. Also watch the speed of the rotors. As Von Doom initially leaves the chopper, the blades can be seen rotating slowly. When the Silver Surfer later rises out of the hole, the blades have sped up considerably.
(at around 5 mins) When the team boards the plane in the beginning, they are shown on a double-aisle wide body jet. Yet when we see the plane take off, it is shown as a Boeing 737, a single-aisle narrow body jet.
Whenever Ben turns into the Thing, his voice changes as a result of the change. Johnny's voice does not change at all when he temporarily adopts Ben's powers.
(at around 48 mins) When Johnny throws beer over the dart board to extinguish the flames it is seen stained, but is then seen completely clean seconds later in the next shot.
(at around 27 mins) When the Surfer approaches NYC, things begin to lose electrical power. At one point, a construction worker is shown confused when his jackhammer stop, and with good reason. The jackhammer is pneumatic, and would continue to run as long as there was air pressure in the tank. Even if its diesel-powered compressor (which doesn't run on electricity) stopped for some reason, the hammer would still run for several minutes.
(at around 45 mins) After the Silver Surfer creates the huge hole in the Thames river, the bed is shown to be empty both upstream and downstream. But the entire river from upstream should have been shown to be emptying down into the hole.
The American army would not be allowed to oversee operations in foreign countries (London, England or Black Forest, Germany).
General Hager and the rest of his "army" troops are shown wearing blue Navy camouflage uniforms in several scenes (such as Black Forest and Research facility). Captain Raye is even wearing Navy Rank insignia on her collar. United States Army personnel, especially generals, have no reason to don Navy uniforms.
(at around 1h 12 mins) When the Four fly in the open Fantasticar at incredible speed, their hair is moving as if disturbed by a light breeze only; this is especially obvious with Susan Storms hair.
(at around 4 mins) When the Four are on the plane getting seated, there is a woman next to Ben Grimm's seat. When Grimm sits down, she's gone. Immediately afterwards, she's back in her chair. However, she simply stood up. Pay attention and you'll notice that her purse is still visible on the right side of the frame.
Fantastic Four (2005) established that only the suits which they were wearing when exposed to the space-cloud's radiation, can accommodate their abilities. However, in this movie, all clothing of Reed Richards elongates with his body. Being a great inventor, he could have made extra clothes to suit his abilities.
(at around 18 mins) In the beginning at the airport on the news channel, the anchor woman talks about Reed and Sue's wedding, and it shows a picture of Reed and Sue, Sue has brown eyes in the picture. They also show the same picture of Sue and Reed on the front page of the newspaper.
(at around 8 mins) During Reed and Johnny's "let's have a bachelor party" conversation scene in the lab, one of the shots is a mirror image (identifiable by reversed text on one of the book covers on the desk.) It is a flipped shot of him pushing away from the desk on the other side later in the scene.
(at around 38 mins) The helicopter that brings Dr. Doom to the glacier (for his first meeting with the Surfer) is heard to have the classic "rotor chop" sound. Yet the rear rotor is shown to be enclosed, a design which eliminates the "chop-chop" sound.
(at around 19 mins) During the first aerial shot of Sue and Reed's wedding you can see the shadow of the helicopter used to film this shot. The shadow is different from the helicopter in shot because it has the spherical shaped camera on the front.
(at around 56 mins) When setting up the machines for the tachyon pulse in the Black Forest in Germany, Thing comes across a bear which is in fact a grizzly. The grizzly is native to North America, and only occasionally found in Europe, certainly not in Germany's Black Forest.
During the final battle against Doctor Doom in southeast Asia, the scenery constantly switches between the cities of Hong Kong and Shanghai, China.
(at around 35 mins) While Johnny's being examined after interchanging powers with Susan, we can see two incorrectly positioned radiographs on the wall: a posteroanterior chest view that's flipped horizontally, and a simple abdominal one that's flipped vertically.