Mind your P's and Q's

So if you've been following the kind of news I do you are surely aware of the Large Hadron Collider built by CERN. This news brought back an old memory from my college physics class. I has this Scottish professor who would occasionally make mistakes in front of the class. One day he was lecturing on hadrons. He thought he wrote hadron on the board, but switched the r and d around. Someone in the class mentioned to him that he had misspelled the word to which he replied, "I'm Scottish, I make spelling mistakes." He went on with his lecture for a few minutes as we laughed a bit now and then about the word he had unintentionally written on the board. He eventually came back to what he had written, realized what it said and became very embarrassed. He promptly dismissed class and the incident was never spoken of again, at least by him. I have a friend from that class that I still bring it up with on occasion.