Anything that can go wrong, will; at the worst possible moment.
An example of Mrs Murphy's law:
Your driving to work and your running late.
Not only do your brakes fail, but you have just made it over a hill, and it's a long way down.
Your design project that counts for a third of your year 12 subject score is almost finished after 13 weeks.
Not only has your computer died and refuses to start again (even in safe mode!) you where only 5 mins off printing it out and handing it in later that day. There are no extensions!
Auld wan in pink who does mad poetry at Electric Picnic, tells stories in mad places and does the matchmaking at the Father Ted Festival. Mad buzzer for an auld wan.
Saw Mrs. Nellie Murphy spinning yarns at The Bog Cottage when I was off me tits.
Mr Murphy is a god among over men, he is skilled in all departments, including having sexual intercourse with your beloved family members, he is the king of clowning, and ruler of skids, he runs the swatting in his channel making fun of losers.
Friend 1 "Hey man did you see John this evening he was being such a MrMurphy!"
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.