What are some common mistakes students make with the net electric charge?

Most compounds are neutral; of course, all matter is neutral. In some instances, however, we include a formal charge descriptor in the fomula. Ozone, ${O}_{3}$, and ammonium ion, $N {H}_{4}^{+}$ are examples.
In ozone, the central oxygen is conceived to have a positive charge, and the terminal oxygen a negative charge: $O = {O}^{+} - {O}^{-}$. Around the leftmost atom there are 8 electrons (hence neutral), around the central atom, there are 7 electrons (hence formally positive), and around the rightmost oxygen, there are 9 electrons, hence its negative depiction.