Is a multivitamin overdose possible?

Yes. A person can take excessive amounts of fat soluble vitamins, especially vitamins a and & d to have toxic effects.
Yes. In addition to toxic levels of fat soluble vitamins, multivitamins also have high levels of ions and heavy metals that are not needed unless you have a known deficiency. I recommend that the average patient, with no know deficiency and anything close to a normal diet, should never take a multivitamin as an adult.