Even worse is when people read the instructions yet insist they know better. I'm talking about the types who will take 8 of a pill when the instructions say 4 because "it should make them better twice as fast". Sad to say, but I've learned you usually have to make whatever you sell idiot proof because the majority won't read the instructions. Of those who do, a good number won't follow them anyway because "it doesn't sound right". In their defense, a good number of instructions are either poorly written, or yes, just plain wrong.
Oh, and it could have been worse. He could have bought the NOS engine cleaner thinking it was a drink.