Michaelis has Schulz and a date drinking Diet Coke in 1970, when it wasn't introduced until 1982. They were probably drinking Tab.
He also notes that the Schulz family baseball diamond got lights in the late Fifties, at a time when they were supposedly new even in the major leagues, but most major league teams had had them for 15 years or so by then.
In addition, at one point Michaelis misspells "elicit," putting in an extra L.
Other than that, though, the book is well worth reading. The decision to punctuate episodes of Schulz' life with relevant Peanuts strips was brilliant, and the portrait of his first marriage is chillingly three-dimensional.