User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-10218971-20130713012345/@comment-3146930-20130713012910

Well The Lost Hero and The Son of Neptune both build up the fact that whenever the two groups meet, they fight. Not to mention that Percy decided to tempt fate by saying:

"If things went right, this would be the best day of his life."

He's a demigod, things never go right. So I pretty much saw it coming, but that's just me.