Continuity mistake: That must be some damn good string that Mercedes wraps around her finger - 16 years and it's not even dirty or frayed. (00:15:00 - 01:51:15)
Factual error: The imprisoned Count is crawling through a tunnel with his neighbour prisoner leading the way. His neighbour prisoner has a lit candle showing the way, but when he breathes out, the candle goes out but the 'light' remains. You can clearly see smoke rising from the candle before the scene cuts away. (00:43:25)
Revealing mistake: About seven seconds after Mercedes screams, "No!" at the climax of the movie, there is a shot of all the characters involved in that scene. If you look out the door you can see a white car drive by. (01:59:40)
Continuity mistake: In the scene where Mercedes confronts Edmond for the second time and he finally admits who he is, she makes a point of showing him the string still tied to her ring finger. Two scenes later, it is also visible when she is still asleep in bed. However, in the scene after she has been shot and everyone is tending to her, her left hand is shown several times and there is no string. Considering her statement that it would, "never leave my finger," and her later reiteration of that in addition to, "and it never has," for the 16 years she was separated from Edmond, it is strange that it has suddenly disappeared.
Visible crew/equipment: Very near the end, when they are doing a pan-out of the deserted building, there are three of the five characters standing in plain view. The other two are in the building, one is helping the other cope with a wound. When they do the pan-out, look through the bottom right "window hole." There is someone standing there, and it can't be any of the five people mentioned. (02:05:35)
Factual error: When Edmund Dantes is promoted to Captain, we clearly see a brass diving helmet on a shelf in the office. Augustus Siebe's hard hat diving dress wasn't invented until 1819, and didn't appear as in the movie until at least 1837.
Continuity mistake: In the scene where Edmond is running from Fernand, he knocks over a bust. In the next shot of the floor, the shattered pieces are gone, but the unbroken head is still on the floor. (00:24:10)
Factual error: Napoleon continually addresses the British officer on Elba as 'Lieutenant' Graypool despite his displayed sergeant's stripes. Napoleon is a professional soldier who has fought the British and lived with them for some time - he is unlikely to be confused by British rank insignia.
Other mistake: During the final scene on the cliffs of Chateau d'If, the Count, Mercedes, and Albert are standing on the edge of the cliffs and you can clearly see aeroplane contrails cutting through one of the clouds. When they cut back to the sky, they are gone. (02:05:55)
Continuity mistake: When Edmond goes to Fernand for help, they end up fighting in something similar to the living room (it's quite a big room). After the police take Edmond away, he throws his sword, but it hits the fake wall that is supposed to be the living room behind him and it's easy enough to tell that the room behind him is not real.
Revealing mistake: Very near the end of the movie, when Edmond and Fernand are about to start fighting, a power line is visible in the background as the camera pans over from Edmond to Fernand's back.
Continuity mistake: After Mercedes gets shot a handkerchief is put on her shoulder but there is quite a bit of blood still visible. In the next shot the blood is gone, but reappears later. This happens several more times throughout the scene. (02:01:45)
Continuity mistake: Shortly after the knife fight with Jacopo, the main character Edmond jabs his knife in the sand beside Jacopo. In the wide shot immediately afterward, the knife is missing from the sand. (01:04:05 - 01:05:00)
Factual error: During the opening credits, Fernand Mondego is tipping back a bottle of booze on the rowboat. The bottle is quite out of place for 1814, as it shows modern "makers marks" on the bottom. Glass bottles during the French Revolution would not have had these markings; they did not start appearing on glass bottles before the middle of the 19Th century. (00:01:45)
Continuity mistake: Edmond Dantes has just spent several years digging a tunnel through the rock walls of Chateau d'If. Yet when he washes up on the beach, a close-up of his hand shows us perfectly clean, manicured fingernails. Salt water may wash your hands clean, but it certainly won't trim, file and repair broken fingernails for you. (01:01:35)
Visible crew/equipment: In the shot where the rat is being lured into the cell by Dantes and the priest, right before the cup is pulled away and the trapdoor is shut you can see a crewman's booted feet for a brief moment. Presumably to keep the rat going in the right direction. (00:44:20)
Continuity mistake: The Count of Monte Cristo asks Mercedes to dance a waltz. What they then dance is nothing like a waltz.
Other mistake: After the knife fight and the Captain giving Edmond the name Zatarra, the scene cuts to the ship entering a port and the words "three months later" are shown on the screen. By the movie timeline it should have said "Three Years Later." (01:05:20)
Continuity mistake: In the late scene where Edmund and Fernand are fighting, Edmund thrusts his sword through Fernand's right side. Later in the death scene, the sword is in Fernand's left side.
Other mistake: In the dancing scene that takes place toward the end of the film at the home of Fern and Mercedes . When the Count of Monte Cristo asked Mercedes to dance with him, there is a close up of Mercedes' face where you can see that she is wearing contact lenses. There a few more scenes with her where it is very noticeable. (01:31:55)
Answer: I believe it's just because there is a dirty smelly man that she doesn't recognise visiting her ailing grandfather, she just wants to keep an eye on things.