In the morning after the couple have slept outside, Kelly goes to her VW car, which has a front license plate. Throughout the rest of the film, it doesn't have one.
When Kelly connects Ben's hard drive to look what's inside, you can see that she only connects the AC cable and the files are already open on the computer. She needed to connect the USB cable to connect the hard drive to the computer and she needed to browse the hard drive to start any file.
When Ben and Kelly are shopping, Ben picks up the tent case like there's nothing in it.
When Kelly is unpacking and placing clothes in the drawer, the dresser moves about 8 inches while she is looking away. When she notices it and looks down at the carpet, she sees the indentation where the dresser leg was, but an elongated slight indentation is missing which the leg should have created by being dragged on the carpet.
About 2-1/2 minutes into the movie, as they are setting up their equipment, the laptop computer in the background wouldn't have existed in 1973.