1. If you have prepared a really cool warmer, everybody will be late for class.
2. If you have planned an important pairwork activity, an odd number of students will show up.
3. If you give your instructions so clearly that nobody can misunderstand, somebody will.
4. The student who misunderstood the instructions will explain them to everybody in his/her team.
5. Photocopiers always break when you have an important test.
6. The enthusiasm the students show for a worksheet you created is inversely proportional to the effort and creativity you invested into the worksheet.
7. No technology will work properly inside the bounds of a classroom.
8. The vocabulary the students memorise is inversely proportional to its frequency and usefulness.
9. When you are being observed, all good students stay at home.
10. The students will remember the exceptions, but not the rule.
11. When explaining grammar, you will only be able to remember cliche examples from your old secondary school books (such as If you hurry up, you will catch the train, or Look at the clouds. It is going to rain.)
12. Words misspell themselves when you write them on the whiteboard.
13. Whenever you spill coffee at home, it will land on a student's homework.
14. If you slip and fall in class, all your students will be present.
15. If you slip and fall in the street, at least one of your students will be present.
Well, I got as far as number 15. It is your turn now. Your suggestions are welcome.