When Abe is about to reveal to Aaron how the time manipulator works, they are standing in front of the white truck. The four-way hazard blinkers are blinking, then the camera cuts to another one. At that point, the blinkers are off. Then they are on again with the third camera change.
When Abe is first telling Aaron about the box, he starts by explaining that the Weeble was accumulating years worth of protein build-up every few days. He later explains that anything in the box must experience subjective time. That is to say, to go back one minute, you must spend one minute in the box. This means that the absolute most amount of time the Weeble could have experienced is only twice the amount of time the box has existed and been turned on.
The route back to the A end is impossible if it curves parabolically. Since the shortest distance between two points is a straight line, a curved trip would require being in the box longer than it has been on.
Aaron could not have set up his fail safe without either telling Abe or Abe dying when trying to use the first box. For Aaron's fail safe to go back further than the first box Abe ever set up, he would have had to shut down and restart Abe's box after setting his own up in the separate storage unit. That means if Abe had tried to go back without knowing, he would have stayed in the box longer than it had been on so he would be getting out on the box's trip back to the B end. Even if this had not killed him, he would have noticed the time discrepancy.
During numerous takes the director, Shane Carruth, mutters
"cut" under his breath. According to the DVD commentary, this is due to their extremely low budget which did not allow them to "waste" film. Carruth notes that a total of 80 minutes of usable footage was shot; the final film is 78 minutes.