Continuity mistake: When the villains get the last of the plutonium out of the cache, we see one villain shoot the chain suspending a concrete cap; causing it to fall and close the hole. A short time later, we see Maggie exclaim that the plutonium is gone. The cap could not have been lifted because all the miners were taken away, yet the cap is gone and we can see into the cache.
Continuity mistake: When the door gets blown in the room where Arnold Schwarzenegger (with a bag over his head) was strapped to a chair, Arnold and the chair fall down. From the shot behind him, you can see his right hand gets freed. Moments later when they help him back up, both his hands are strapped again.
Plot hole: The seaplane should have a watertight hull so the two jumping out the bottom makes no sense when they should of used the same exit at the rear of the plane that they loaded the jet skies with.
Continuity mistake: When the shootout starts in the airport, the Expendables shoot some bad guys and duck behind a service kiosk. We see Chuck Norris shoot one villain from the second floor behind and above the Expendables. Moments later, when the Expendables advance into the airport, they shoot several bad guys that are on the same level Chuck Norris should be on. Chuck Norris apparently disappeared.
Continuity mistake: In the opening chase with the seaplane the fuselage changes shape from being pointed at the tail end to being square to load the jet skies and as they crash through the exploding bridge to being pointed again.