Andy Hunt and Dave Thomas once wrote a debit card transaction switch. A major requirement was that the user of a debit card shouldn't have the same transaction applied to their account twice. In other words, no matter what sort of failure mode might happen, the error should be on the side of not processing a transaction rather than processing a duplicate transaction. So, they wrote it on their shared whiteboard in big letters: Err in favor of users.
It joined about half-a-dozen other maxims. Jointly, these guided all those tricky decisions you make while building something complex. Together, these laws gave our application strong internal coherence and great external consistency.
Source -Dave Thomas, The Pragmatic Programmers