In what circumstances could a president serve more than 8 years in office?

There are no circumstances you cant run for more than 2 terms in office

This can happen if a person (most likely the Vice-President) takes over for a president who can no longer be president. If this person serves two years or less of the last President’s term, he or she may serve for two full four-year terms. If he or she served more than two years of the last President's term, the new President can serve only one full four-year term.