"Murphy's nartial laws" joke

Murphy's Laws of Martial Arts
Ten scientific principles that apply to the study of all martial arts:
The wimp who made it through the eliminations on luck alone will suddenly turn into Bruce Lee when you're up against him.
The referee will always be looking the other way when you score.
The day you leave work early to make it to class on time, the sensei will be sick.
The sensei will only use you during demonstrations for joint-locking techniques.
If you have to use your training in self-defense, your attacker's father will be a lawyer.
After a flawless demonstration, you will trip on your way back to your seat.
After years of training without a single injury, you will pull a groin muscle the night before your black belt exam.
In an otherwise vacant locker room, the only other person will have the locker right next to yours.
No matter how many times you take care of it before your promotion exam, you will invariably have to go to the bathroom when it's your turn.