Murphy's law is a saying: 'If a thing can go wrong, it will'. It sounds like a joke, but in fact is a very useful maxim to bear in mind when you are planning any project, from a party to a major operation. One of the main mistakes that we tend to make in planning is that we don't allow enough of a 'margin of error' or have enough alternative plans for when something goes wrong. If you keep Murphy's law in mind and assume that whatever is able to break down, get lost, malfunction etc, will do so at some point, you can also devise a plan to deal with every emergency.

According to one story, the original Murphy was an Air Force engineer who was annoyed by the mistakes being made on a project he was involved in. However, the general principle, that humans make mistakes all the time and you can always expect things to go wrong, is much older than this.