One thing about the Seven Days Battles is the question of why McClellan's intelligence apparatus suggested that Beauregard was at Richmond. In point of fact he was at Richmond, after having been fired from command of Department No. 2. The northern papers had reported it, such as this article in the New York Times dated 24th June 1862. Later they report on the 29th that his army is following, the day before that they reported a considerable portion of it had arrived already. They'd reported that Beauregard may move there a month earlier.
It appears that the Union papers (and hence intelligence apparatus) had detected the start of Bragg's change of base from Tupelo to Chattanooga, but combined with the appearance of Beauregard in Richmond had put two and two together and made five. It looked like a detachment of Beauregard's army was going to Richmond.
Now, McClellan's apparatus apparently detected the movement of the force under Holmes from the Department of North Carolina and Southern Virginia. Under questioning prisoners from Holmes claimed to be part of Beauregard's Army (matbe Pinkerton's interrogators asked leading questions....). They too put two and two together and made five. The result was that McClellan was getting intelligence from several directions, although he does not necessarily believe it. His communique to Edwin Stanton states he would "have to contend with vastly superior odds if this were true." (emphasis mine). Of course there is sufficient weight of evidence to give some credence to this at the time.
McClellan was wrong about Beauregard's army being in Richmond. However, there was sufficient evidence to that effect that pointed to it that the conclusion was reasonable given what was known at the time.