When Barney is firing his sidearm at incredible speeds during the final shootout, the sounds of the gunshots come much faster than he can be seen pulling the trigger. Furthermore, he fires far more rounds than the Colt pistol can hold between reloads.
When Ross and Christmas are investigating the island, Ross' goatee changes color from black to grey. When he is avoiding the troops in the street it is clearly grey, but when driving in the truck it is again black. Later on, on the same island it changes to grey once again.
The final time that Barney Ross gets into the cockpit of the plane, he gets into the seat while wearing a tactical vest. When he is shown from the front, he no longer has the vest on, but is wearing a t-shirt.
When Barney Ross and Yin Yang stop by a traffic light just before the car chase with Gunner Jensen, the car behind them slows down and stops. When the camera changes angle the same car is seen from a distance slowing down and stopping again.
After Barney tells Gunnar he can't trust him no more, Gunnar walks away. As he does he lifts his right hand to point at Barney. When the camera changes angle he's pointing at Barney with his left hand.
While Sandra and Barney are talking in her truck, a large piece of plywood is seen bouncing in the bed of the truck through the back window. In a shot of the back of the truck as it is driving away, the plywood is gone.
When Barney Ross and Lee Christmas arrived at the island for the first time to investigate, Lee had a smaller prime lens mounted on his camera. But then in the city he had a bigger zoom lens attached . He didn't seem to be carrying a bag with him so there's no logical explanation on how he changed lenses.
When Barney sits down to let Tool finish his tattoo, the letters on his back only spell "EXPEND". It then takes Tool about 15 seconds to finish the word "EXPENDABLE", in between talking to Barney and Lee who came after they started.
During the final knife-throwing competition between Christmas and Tool, initially Christmas is holding the throwing knife by the blade and then a split-second later he can be seen holding it by the handle.
When Barney and Lee are attacking the docks, the plane's course switches 180 degrees between shots. One moment the town is at their back, the next the town is at their front, only to then have the town at their back again.
When Barney is holding onto the doorway of the plane, he climbs inside and rolls onto the floor, the door is clearly still open behind him as he makes his way to the cockpit. The next scene is of the plane flying over the water and the door is clearly closed.
At approximately 35 minutes, when Christmas leaves the truck to get a better look around, Ross says not to lose his hat. Christmas takes it off, doesn't give it back or toss it, but when seen walking away, he no longer has the hat.
In the basketball court scene when Christmas is fighting Lacy's boyfriend, he shoves a basketball onto his chest and proceeds to stab it. A chest protector is clearly visible under the basketball and covering the stomach.
Several problems with the Fly & Die scene: Christmas turns left and fires the flare slightly out from the plane, but in the reverse shot the flare is coming from the right side of the plane. A flare is slower than the plane, meaning that firing it backwards would still make it fly in the direction of the plane, only slower. The flare passes through the cloud of flammables, but doesn't ignite it. The explosion stops at the end of the pier for no reason, even though the highly flammable cloud extends beyond the pier all the way to the plane.
The silver badge on General Garza's right breast is actually a United States Army Combat Action Badge. It is awarded to non-infantry soldiers who have actually engaged an enemy force in combat, and worn on the left breast. Since it would be nearly impossible for a foreign general to earn a CAB legitimately, the costume department most likely simply picked out a random military badge.
Once Barney and Christmas are taken near the presidential palace by Sandra and they get out of the car, a sign is shown that reads "Área Militar, Prohibido Passar." The word "passar" has a spelling mistake and it should be "pasar."
As Ross and Christmas are leaving the island on their first trip, they shoot at the docks which are soaked in gasoline. In one shot we see the explosion clearly expanding horizontally several feet off the pier. On a bird's eye view shot we see a boat next the the burning pier clearly not affected by the blast or the fire.
The goof items below may give away important plot points.
Ross cuts off the guys hand and head of bad guy #1. He then throat stabs bad guy #2. On the left side of the frame, the first guy is laying on the ground with his head re-attached and his face is moving.
James Munroe has Sandra at gunpoint as a hostage, Barney Ross approaches them with left hand supporting gun hand at gun grip, in next shot he is supporting gun hand at wrist, then again at grip in the following shot.
During the final battle just after the tower nearly collapses on Hale Caeser and before Barney Ross destroys the helicopter, Munroe is running with Sandra. As the shot of them running pans along, you briefly catch a partial glimpse of someone wearing Bermuda shorts and holding what appears to be a camcorder.
When the Expendables return to the island of Vilena to rescue Sandra, they miraculously 'appear' undetected outside Garza's compound. As Barney's plane is subsequently seen to be berthed at the dock, they must have left it there on arrival. Based on the first visit they could not have achieved this, the dock no longer existed. They blew it up as they left after the first visit.