Simon Cowell -- 'The Voice' is Haunting Me!

Simon Cowell's big Hollywood party for "X Factor" was crashed last night by Christina Aguilera and the rest of the "The Voice" judges ... who rode in on a huge bus.

Poor Cowell was talking about tonight's premiere of his show ... when he was upstaged by a double-decker bus with a massive 'Voice' billboard on the side of it -- hysterical stuff.

Simon was a good sport about it though -- or really stupid -- since he pointed out the bus to cameras and then laughingly said, "That was timed."

Simon's misfortune continued later ... when we asked him about "The Voice" going head-to-head with "X Factor."

Our camera guy said, "May the best TV show win" -- and Simon made the poorly worded response ... "No. Let the 'X Factor' win."