It was the same as a story I read in facebook, when a teacher wrote the multiplication table of 9. It was written like this :

• 9x1 = 8
• 9x2 = 18
• 9x3 = 27
• 9x4 = 36
• 9x5 = 45
• 9x6 = 54
• 9x7 = 63
• 9x8 = 72
• 9x9 - 81
• 9x10 = 90

The students laugh because there was a mistake on the first statement but they forgot that the remaining statements are correct.