That's why over/under lines are usually whatever number and a half, since that's not actually possible to really happen, there will always be winning (and losing) bets.
On the off chance an over/under line is a whole number (or otherwise a result that can possibly happen), and that's what actually happens, it's considered a "push" and all bets are refunded since nobody won (or lost) their bet, kind of like a "push" if you're playing blackjack and tie the dealer. That's not generally a desirable result for the bookmaker. However, they occasionally can make money off the over/under bet anyway by charging a small (and non-refundable) fee for making the bet.