Revealing mistake: When the plane first lands at Bangor airport, the words 'American Pride' on the plane are reversed, like they are reflected in a mirror in one shot as its wheels hits the runway. (00:50:55)
Hamster
Other mistake: In the scene where they are 1st landing at Bangor it is known that there is no electricity at the airport. If you look closely during the landing you will see that the runway lights are on. (00:50:40)
Factual error: When initially investigating the missing people and the condition of the airplane, Brian looks at a screen that gives the airplane's transponder code (or "squawk") as 7777. An airliner would never be squawking that code, since this particular code is reserved for military interceptor operations.
Answer: Think of time as a gear with only one tooth, and think of them as a gear with only one notch. In the past, the one tooth has forever passed their notch and they'll never be carried along in the flow of time again. In the future, the tooth comes along, snags their notch, and they're back in the flow of time.
Phixius