Factual error: When they are at Aunt Meg's the first time, they get word that a tornado has been spotted and somehow they already know its rating. Tornadoes get their ratings from the amount of damage they do. This is determined after the tornado is gone.
Continuity mistake: When they're in the town with the drive-in movie, we see Melissa in her motel room crawling up onto the bed towards her pillow. You can see she has shoes on but no socks. We see her twirling her engagement ring around on her finger, then see her use the remote to turn the TV off. She watches the curtains blow for a minute and then she walks out of her motel room. She has on white socks. Then as she runs to grab Billy's hand, she has on black socks.
Suggested correction: They got dirty.
They wouldn't have gotten so dirty that they would be black. At the most, they would have turned a light brown colour.
Other mistake: When the twister hits the drive in theater, the sirens are wailing, but nobody reacts. Only when Jo screams at the waitresses they start panicking and run. The movie is set in Oklahoma, smack in the middle of tornado alley. The people there know what the sirens mean, they sure don't need no out-of-town big-shots to tell them to get under cover.
Suggested correction: Sirens go off so many times that true Okies tend to ignore them. We might get nervous if the cable goes out.
Do true Okies also run screaming the second an out-of-town big-shot yells at them? Sorry but that argument just doesn't compute. People ignoring fair warning simply isn't a thing in the movie. The main plotline is people not getting warnings soon enough.
It's also possible that they did hear the siren, but unlike the people at the drive-in who could actually see the tornado tearing the screen apart, the people at the snack bar might not have been able to see it (especially with an awning over the windows). They were probably at most confused as to what was happening and it wasn't until Jo actually said to get underground that they realised it was real and was actually happening.
Other mistake: When the Tree gets blown under the red truck and they get stuck on it. Why don't they ever put the truck in 4WD? You can see the front differential and 4x4 markings so the truck definately HAS 4WD. That particular year of truck doesn't even require you to get out and lock the hubs.
Suggested correction: Character mistake more than anything. In a panic situation you don't tend to think of the obvious solution. The other issue to is that even if the truck was in 4wd that it might not have been enough. Keep in mind that the rear tires did burnout on a few attempts so the issue wasn't that the wheels weren't stable on the ground but that the tree trunk might have been too big to get the wheels over it by driving across (especially from the angle they were at).
Other mistake: When Jo readies Dorothy IV on the back of the truck, she switches it on, but she never releases the rubber catches holding the lid in place.
Suggested correction: The rubber catches weren't needed to open the machine; they were only there to secure the entire lid in place. As seen when Jo is first showing how Dorothy works, the lid could be opened manually or automatically, depending on which button you pressed. As long as the pack was "set up" (sirens on and buttons activated), the pack would open automatically when close enough to the tornado, as shown when the pack lid opens at the end.
The way I understood it, the rubber catches were there to secure the lid in place during travel and had to be released for the automatic opening mechanism - the existence of which you are entirely correct about - to open the lid. That makes sense entirely, if you are bouncing over rough terrain, you would not want to load gears or servos with the considerable force spikes thick acrylic panels would impart on them were they not otherwise secured.
Stupidity: In the opening, it's surprising that Jo and her family have to actually go outside and run even a short distance to get to their cellar. Farm houses, especially in tornado prone areas like rural Oklahoma, would typically be built overtop of the cellar, which would be accessible from inside the house. Furthermore, Jo's cellar door seems to be secured with what seems to be little more than a latch much like those used for restroom stalls! Not very practical for a door intended to protect the interior from outdoor elements.
Suggested correction: Their storm cellar is made of an old septic tank, which is very common to do. It's half buried into the ground so it provides maximum protection, unlike under the house because an F5 tornado would simply rip the entire house away, removing all the protection the cellar would have had. Also the door has several latches, not just one.
A cellar under the house would likewise be buried into the ground, often more than halfway, so it would offer just as much protection, if not more. Looking at the scene again, the door has two latches, both the same size and woefully insufficient.
Continuity mistake: During the movie theater scene all the vehicles except the bus and red truck are destroyed, but during the F-5 scenes all the vehicles are there.
Suggested correction: The vehicles remaining after the drive-in scene are Bill's red truck, Beltzer's van, Joey/Lawrence's Chevy truck, and Dusty's bus.
Continuity mistake: After the first tornado when Jo's truck is destroyed, you see Bill's truck with a shell on it. When their group is leaving the scene, the shell is gone and one of the Dorothys is in the bed. Where did the shell go?
Suggested correction: ANSWER; As some time elapses between the tornado aftermath and leaving the scene, there are plenty of reasons why the shell moved.
Continuity mistake: When the tornado at the drive-in makes the sign with the titles of the movies fall, the arrow on the upper part of the sign is pointing to the left. However, when the upper sign detaches and heads into the garage, the arrow is pointing to the right.
Suggested correction: It's during a tornado. Things are going to be blown every which way.
Other mistake: While in Wakita, in northern Oklahoma, the crew see the report of the F-3, and it was 30 minutes away. They meet the tornado in Texas, several hours away.
Suggested correction: The tornado is actually 30 miles away, not 30 minutes. Quite a difference for time.
Other mistake: Near the end after Bill and Jo have jumped out of the truck, their crew sees the truck and Dorothy go up in the tornado, they are receiving telemetry, then one of them notices the tornado shift direction and the crew still tries to notify Bill and Jo by radio, knowing they're no longer in the truck nor have radio communications.
Suggested correction: Jo still has her headset on. The crew can probably communicate with her that way, but she may not have been able to hear them since they were so close to the tornado in the cornfield.
Suggested correction: The scale back then was based on the size of the tornado, it's only more recently it is based on damage. So during the time of the movie, the scale was being used correctly for size not damage.
The Fujita scale was introduced in 1971 and was in use during the 90's when this film came out. The Fujita scale measured the damage caused by a tornado to man-made structures after ground or aerial surveys, it was not a measurement of tornado size (an F5 tornado is a tornado that's rated on the Fujita scale). It is true the Fujita scale was replaced by the enhanced Fujita scale in 2007, but that was only to align the ratings to the damage better, it did not change rating tornadoes from size to destructive powers.
Bishop73