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 seeJohn this evening he was being such a MrMurphy!"