When Victor went back to the Pet Cemetery to retrieve Sparky to bring him back to life, you see some flowers on top of where Sparky was buried. In the next scene, when Victor starts digging, the flowers moved to the side with no indication of Victor bending over to move them.
During the classroom scene when Mr. Rzykruski is explaining the mechanics of lightning, his diagram shows positively charged clouds above the ground, which is shown as being negatively charged by electrons. The diagram should contain negatively charged clouds (containing a surplus of negatively charged electrons) over the ground which should not have any charge.
The goof items below may give away important plot points.
Toward the end of the film, when Sparky has been hooked up to all of the jumper cables, Victor places two cables to Sparky's neck bolts. When the charge seemingly doesn't take, Victor removes one cable, not two. When Sparky reawakens and stands up, the second cable is not attached.
The sea monkeys would not explode in the salty popcorn. Through osmosis, water moves towards a saltier area to create balance. A freshwater creature in a saltier environment, like the sea monkeys, would shrivel up and die of dehydration as the water moved out of its body. It is a saltwater creature in a freshwater environment that would explode as the water moved into its body.