Watch the episode where Nick gives Lindsay some pot that she smokes before she has to babysit. When she's reading the encyclopedia article on marijuana from the M volume, in some shots she's reading the last few pages of the book. But because marijuana starts with "ma" it would mean the article would be in the front of the book.
I just saw that episode the other night and I noticed the same thing. It also looked like she was supposed to be reading it for a long period of time and I doubt that there was THAT much info on it. Unless it was the pot making her read slow.
I saw the same thing, but I don't necessarily believe it was a slip up. Some encyclopedias are not organized so neatly as to be letter-discrete (for lack of a better term), e.g. The first volume might be A - BE. The volume in question could be KO - MA, for example.