Continuity mistake: At the end where the building explodes, everyone stands up from the pit. Burt is standing up in one shot, but in the next, he is kneeling down.
Visible crew/equipment: In the shot where the camera examines the hole in the wall and car made by Burt shooting the shrieker. You can see the shadow of the cameraman on the wall and his reflection on the car (near the top right corner).
Continuity mistake: The scene where Fred is trying to rope the ostrich he gets pulled for a couple of feet and looks up at the bird. If you notice his shirt is dirty from being pulled by the bird. When he gets up to meet the taxi his shirt is magically clean.
Other mistake: When Kate is introduced by Senor Ortega, she is introduced as Kate Riley, but the credits list her as Kate White.
Continuity mistake: When Earl enters the barn after being hosed down with CO2, the amount of vapor as he warms up varies from shot to shot with it almost not appearing to it heaving pouring off his clothes.
Continuity mistake: While driving in search of the graboids, the blue square blip on the seismometer represents their truck, and it's moving. However, when they are using the RC truck, the seismometer shows the blue blip is still moving, but the truck is now immobile.
Continuity mistake: Earl and Kate are talking about dating at the end and walking with Burt and Grady towards the hole in the ground. In the first shot, it appears to be overcast. Suddenly the sun appears on everyone.
Visible crew/equipment: After Burt shoots a hole through the wall, the camera shows the hole and backs up. Its shadow is on the right of the screen.
Visible crew/equipment: Right after the Graboid jumps onto Earl's truck, they panic and reverse really fast. There is a shot looking through the back window of the truck. The camera reflection is fairly obvious.
Revealing mistake: When they are using the broomstick in front of the Shrieker, there is a seismometer in the background. At the very start of the scene, the screen reads "Kathy's Lab Template," and then the screen flickers before showing the seismometer as normal. This indicates that this is just playing back from a pre-recorded VHS tape.
Visible crew/equipment: When Burt Gummer is shooting at the Shrieker, in the top left corner of the window you can see a crewmember quickly bash the window causing it to shatter.
Factual error: After Grady and Earl wreck their truck, Burt, Earl and Grady have a radio conversation. At one point Grady interrupts Burt's transmission cutting off his voice, but making it appear Burt can hear him. It's not possible to interrupt a transmission while the other person is also transmitting even though they're using repeaters which is why Burt's voice was cut off in mid sentence. The only way you can transmit and receive at the same time is if you're using full duplex. Burt's voice was cut off, therefore they are not using full duplex.
Continuity mistake: When Grady and Earl wreck their truck, Earl picks up the radio mic to call anyone at their base site. He is talking into the back of the mic in two shots, then correctly in the next shot.
Continuity mistake: When Earl and Kate are talking (about her Playboy photo) while stuck on the roof, she is wearing her ring on her left hand (it was on her right earlier in the film). In the film's timeline, it is minutes later that the ring disappears.
Visible crew/equipment: When Ortega introduces Kate to Earl and Grady, she turns from looking at Ortega to the others, and we can see the lighting rig reflected on her sunglasses.
Factual error: When Grady and Earl are trying to sleep on a boulder, there's a very large moon in the sky. It's clear that it isn't the actual moon. None of the craters or other markings correspond.
Answer: It's a 50 caliber Grizzly Big Bore rifle. They are real. Just put "Grizzly .50 Big Bore" into your search engine and you will find plenty of sites.
Grumpy Scot