When Micheal and Ellie are talking in the GTO, the passenger side window is up except for camera shots from that side of the car. When Ellie opens the door to leave, the window is down. In the next cut, as she walks away from the car, the window is up.
The goof items below may give away important plot points.
The device was never unplugged from the outlet after Michael, Vinnie and Ellie destroyed the electrical tower. In fact, Michael and Vinnie were blocked from re-entering the school by police.
Later, when Ellie goes to recover the device and finds Sherman at the lab, he plugs the device back into the same outlet.
When Michael, Vinnie and Ellie are showing the device to Bob in the lab, Bob connects it to a battery-block. Shortly thereafter, the device causes the battery-block to melt down. Later, when Bob is pulled into the time-warp, the battery-block is shown undamaged.
The device appears to draw more power than can be supplied to the town, making all the lights in town flicker. Even when connected to an A/C outlet, the device could only draw as much power as the circuit breaker would allow.