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!