When Robert Neville is shooting at The Family (who are behind a car), you can clearly see a fairly straight line of bullet holes running horizontal across the car. As this happens the tires pop even though it would appear the bullets never hit near them. The scene cuts away from the car. When it cuts back to the car, the bullet holes are gone.
When Neville's first car is damaged, he proceeds to a car dealership to procure a new one. The replacement car is very dusty, and the windshield is obscured. After Neville drives off from the dealership, the car looks pristine, like its just been washed. After he gets back to his apartment and parks in the garage, the car is dusty again.
Near the beginning of the movie where Neville says "They'll be waking up soon" he jumps in a blue convertible and drives up the hill. 2-3 blocks behind him you can see 2 cars drive through the intersection.
23 seconds into the opening of the movie, immediately after Neville puts an 8-track cassette in to play, a man can be seen moving on the sidewalk to the actor's left.
As Neville zooms past the Family members on his motorcycle, no light appears on their bodies in the POV shots despite the fact the motorcycle light is on.
In the opening scenes of the film there is supposedly no electrical power to the city, yet some of the traffic lights are shining red.
A small generator of the size shown in the movie theater would never have been able to power the arc light bulb of the theater's projector, let alone the projector AND the theater's sound system.
When Neville is refueling the generator, he flips the starter switch and gets no response. He then primes the carburetor, and it starts right up. While that may have to be done on an engine that has run out of fuel, it would still have turned over when dry.
Right after Lisa says "I just made my move" there is a cut and in front of Neville is a platter of fresh fruit (apples and grapes). Several years into a post-apocalypse world, fresh fruit would be a nearly impossible find in a large city. However, it's possible this was artificial "wax" fruit, there for ornamental reasons. Even if the fruit was real, Neville could well have obtained it directly from an orchard and vineyard someplace (California has an abundance of both). Plants will continue to grow and produce fruit, even if the people who own them aren't alive anymore.
Though signs of life are sometimes seen -- cars in the background -- the appearance of the young people indicates there are survivors. They just may be scattered or in hiding.
In one scene, Neville and Lisa go into a drugstore and are looking at various types of medications they can use. The world has been deserted for 2 years, meaning no new products would have come in for at least that long. Medicines have manufacturer expiration dates which have been proven to be artificial. Dry pills are stable for many years past the manufacturer expiration dates. This would have been considered as a goof until only recently.
Birth control pills were never kept on open shelves in drug stores. However, they could have been left there deliberately once the war had broken out.
In a city supposedly laid waste, Neville has to resort to running a generator any time he requires electricity. He does so to power his apartment; he does so to power up the projector inside the cinema when he goes to watch the film. But this city with no surviving infrastructure (in the opening scenes, as he's driving around in the red convertible), all the traffic lights are powered up.
When Robert Neville is in the helicopter which is supposedly spinning out of control, the reflection of the same tree can be seen in the copter's windscreen throughout the entire flight.
After Neville leaves the red car at the beginning of the movie, immediately after the zoom out two vehicles can be seen on the highway speeding by in the distance of the upper left field.
When Neville and Lisa ride on the motorcycle it is obvious that the driver is not Charlton Heston, but a stunt double.
(at around 27 mins) Robert shouts out, "Matthias," and it echoes across various empty city views. However, in the last view several vehicles (including a bus) can be seen driving along a highway on the upper far left of the screen.
At 24:46 Nevile fires at the Catapault with a Browning Automatic Rifle (B.A.R.) in automatic mode. The view through the scope shows the cross hairs barely moving even though Neville is firing standing unsupported, a very weak firing position. In reality, the first burst would have resulted in the cross hairs rapidly climbing upwards from the rifle's recoil.
The war footage at about 12 minutes into the film, when Neville enters the elevator and presses the button, was shot with spherical lenses and not properly desqueezed for the film's 2:39 aspect ratio.
There is no way that Neville (or his bag of experimental vaccines) could have survived the helicopter crash, as it exploded upon impact.
Early in the film, when the mutants have attacked Neville, he kills three of them inside the garage, a fourth escapes through the fire. Yet, when Mathias and Brother Zachary speak of it later, Mathias says "Not 3, brother." Zachary replies, "3 brother." He then names one and says two more were shot when Neville brought down the door. But All three were killed before the door came down. (And their blood remains on the garage wall through the rest of the film!)
In one of the early flashback news sequences, the reporter warning people to stay in their homes concludes with 'CONELRAD channel' - CONELRAD was an acronym for Control of Electromagnetic Radiation developed by the FCC during the Cold War, and was abandoned in August of 1963, replaced by the Emergency Broadcast System in the same month. Any 1970s alert would have used the EBS, not CONELRAD.
After Neville and Lisa's night together, Lisa sits disrobed on the bed and says "Well, doctor, you certainly haven't lost your bedside manner." If you watch her face carefully, you'll see her lips never move for this line.
The 8-track that Robert Neville puts into his car stereo at the beginning of the movie can very briefly be made out to be Frank Sinatra's "Strangers in the Night". What actually plays on the soundtrack is Max Steiner's "Theme from 'A Summer Place'".
When Matthias is chiding Zachary for his "Honky paradise" comment ("forget all the old hatreds"), his lips don't quite sync with his words.
When Neville exclaims, ''Damn! It's jammed!'' he isn't actually saying anything, he's simply gritting his teeth in frustration.
When Neville and Lisa are about to kiss in his apartment, the lights go out due to the generator stopping and Lisa says "The Family." If you watch closely her mouth can be seen moving after the words are spoken.
The camera crew is reflected in the black stone pillars when he is jogging along the Los Angeles Department of Water and Power.
When Neville injects himself with the vaccine after the helicopter crash, a "blood pouch" is clearly visible in his pant leg. Also, he clearly doesn't insert the needle into anything.
During the inquisition you can see reflections of the crew in the silvered glasses of the Family.
When Neville is playing chess with Caesar early in the film and The Family are using what appears to be a mangonel-type catapult to sling 'fireballs' at Neville's apartment, one fireball makes it just inside and sets the curtains on fire. After Neville puts it out with the fire extinguisher and turns to put the fire extinguisher back and retrieve the rifle with the nightscope, you can see the ball, now covered with the white extinguisher powder, move and see it is suspended by a string/fishing line.
After Dutch is given Neville's flask of the blood serum, the cameraman's shadow is seen on the ground.
In the opening sequence, right after Neville starts his 8-track, he's driving on Flower & turns left on Wilshire. The very next turn he makes is also a left - from Flower to Wilshire. This is not directionally possible - he would have to go around the block to make the same turn again. You can confirm by looking at the distinctive buildings in the background & noting where Flower curves into Hope St in the background just beyond the 4th street overpass for both shots.
If the Family are so determined to get rid of Neville but don't want to use high-tech means, why don't they just set his building on fire on the ground floor exterior. Neville cannot be awake all night *and* go out patrolling during the daytime.
At the beginning when Neville gets back to his apartment and makes himself a drink, there is unmelted ice in the ice bucket even though he had no opportunity to put it there.
In the lab, Robert puts a test tube in the mini centrifuge and switches it on. Centrifuges need a counter balance in the opposite slot; failing this the high-spinning rotor vibrates erratically causing breakdown.