The other DrC and I are listening to H.P.6 on the CD player in the truck as we drive. We ran across one of those irritating things that says Rowling's continuity editor dropped the ball. Let me set up for you the issue as it appears.
During the summer before Potter's sixth year at Hogwarts School, he and Dumbledore visit a retired professor, one Horace Slughorn, and convince him to return to teaching. This happens just before Harry's birthday on July 31, as Harry subsequently spends his birthday at The Burrow, the home of the Weasleys. Some 5-6 weeks later Slughorn teaches his first lesson and has a variety of cauldrons of potion simmering in the classroom for the students to identify, learn about, etc. One of those cauldrons contained felix felices, the lucky potion.
Some months later Ron says to Harry, something like "Wouldn't it be handy to brew our own supply of lucky potion?" So Harry looks it up and discovers it takes six months to brew. Slughorn simply didn't have time to brew it up from the time he agreed to return until the time the first class on or about September 2. Old Sluggy says, and we have no reason to doubt him, that he's only taken it twice in his life and neither time was recently, so there is no reason to believe he had brewed it up just to "have on hand."
The literary screw-up wasn't him showing students the potion, the error was Rowling gratuitously introducing the idea that it takes 6 months to brew, a factoid that adds little to the story and could easily have been omitted. I suppose Slughorn could have used a time turner to go back six months and start the potion, but that is extremely improbable and entirely unnecessary from a plotting point of view. Nobody puts that kind of energy into lecture preparation.
