When science teacher Bob Roberts (Dennis Hopper) returns from the 1960s, he's wearing the same costume that Hopper wore when he played Billy in Easy Rider (1969).
In a scene where John Stockwell's character is under his car fixing it, Fisher Stevens says to him: "Ya know, I seen this movie where this haunted car flames guys out.", referring to John Carpenter's Christine (1983) in which Stockwell had a major role.
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 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.