Now Absalom had commanded his servants (2Samuel 15)

2Samuel chapter 15 opens with Absalom hatching a plot against David. He stands by the gate of the city and implies to people that he would be a better judge than his father is, and people believe him. When did Absalom rebel against David? I think there are a couple things that could be said in response to this supposed contradiction. One is that when it says David reigned 40 years, it may mean that David reigned 40 years until Absalom took over, although I think that's unlikely. What is more likely is that "after forty years" is rather unclear. "After forty years" of what? After David had been king forty years? After Absalom stood at the gate for forty years? After Absalom was forty years old? I suggested the first was a possibility, but unlikely; the second one is extremely unlikely, although it's what it sounds like in context; the third is a real possibility, but once again, it's unclear. I think you can't base a contradiction on such a vague statement.(See the comments for possible explanation.)

So Absalom goes to Hebron with the excuse that he made a vow of some sort, and David lets him go. However, this is the time that Absalom sets his rebellion in motion, and David hears about it in time to flee Jerusalem. He brings most of his household except for notably a few concubines to look after the house. Also, after some distance, he sends back the Levites and the Ark. There's an exchange in which David is told that Ahithophel, one of David's counselors, is with Absalom, and David prays that God would turn his counsel into foolishness. At about this time, a man named Hushai shows up, and David tells him he can serve him better by staying behind and making trouble for Ahithophel, and communicating stealthily with David through the priests.