According to Jim Recap: No Surprises
Jim has never been able to throw a surprise party for Cheryl because she is one of those people who won't let you throw a surprise party. She either finds out the details or she holds you down and threatens to spit on you (at least, if you are Andy or Dana she does) until you tell her about the plans for the surprise party. So it's just no fun. This year, she does just that and completely ruins the plans that Jim had, so he swears off of all parties. But it's all good - Dana has a new promotion, so Cheryl can go to that instead.
Meanwhile, Jim and Andy can also sneak off to the "Bowling with the Bears" promotion, and who wouldn't want to do that? Of course, Cheryl wouldn't want to do that - especially on her birthday night. But that's not too much for Jim, and sure enough, he and Andy sneak out, with the excuse that a construction emergency came up. They're totally busted when they come in, smelling of smoke and alcohol.
So it's with a certain amount of satisfaction that Cheryl heads off to Dana's promotion party instead. She's even happier (and more upset) when Jim tells her to keep the sitter, because that way he can have an enjoyable night at home without having to keep up with the kids. As she gets to the restaurant and finds out that she went to the wrong one, and she'll have to drive across town to get to the right one, she figures she will settle in for a drink and eat by herself - until she finds a note in her purse that Jim took her last $20 for lottery tickets. It's only then that everyone pops up for a surprise party - they got the best of her this time!
