Other mistake: In the scene where Christine first meets the seer, the seer takes Clay's credit card to charge $60, but in the uninterrupted events that follow, he charges nothing to the card. After becoming frightened, he insists on 'returning' the money, but is later revealed to have taken the money anyway. After having insisted on not taking the money, it would have taken an uncomfortable set of circumstances for the group to agree to let the seer electronically charge the money to the credit account anyway, rather than agreeing on no charge and simply leaving.
Other mistake: When Christine fixes Clay's printer, there's just one small problem. Clay has two iMacs in his office, seemingly connected to an old HP DeskJet printer with a parallel interface. New iMacs do not have parallel ports, so Christine only solved one of Clay's problems.
Answer: Because it's a film and Sam Raimi obviously thought it was funny. But I did also wonder.