When they first escape the lumber camp and fall in the water, it is a water hole in a desert with nothing but sand around, yet when they crawl out they are in a woodsy setting.
When the escape hole is begun, it's dug into the wall so that one would have to step up into it, and put one's legs down coming out of it. Later on, the bottom of the hole is at floor level, requiring one only to straighten up when coming out of it.
When the free humans raid the alien camps, the first shot of the humans breaking into the slave camp through a fence is re-used, even though not all of the camps have fences.
With fires and torches burning in the underground holding cell all the time, the people being held there should have suffocated.
Kelly lost a finger trying to escape the camp, indicated by her missing ring finger on her right glove. However, when she is not wearing the glove, and holding onto Ivan's arm, her finger is in tact.
Obviously fake beards on most of the extras.
Assuming that factory produced gloves have not been made for twenty years and are acquired when someone else dies, the gloves worn by some of the slaves are in excellent condition despite being the inexpensive, cheap to produce kind. This is most evident after the "freedom" speech.
The aliens invaded twenty years ago, yet simple gestures such as handshakes have been completely forgotten. Even though much of the cast had to have been alive before the invasion.