Believe It Or Not
Murphy's Law as applied to programming states:If there is any bizarre set of circumstances that will mess up a program, the customer will eventually find it.
You must try to plan for every possible situation, but you will almost certainly miss something.
A repair dispatching application I worked on assigned a few thousand jobs to a hundred or so employees each day. When the project was almost finished, we installed it and let the customers use it in a controlled test. As we were expecting, we found a few bugs and made some improvements.
We also encountered a situation we could not have imagined even in our late night fantasies influenced by hours of debugging while mainlining caffeine and listening to Pink Floyd. In two different instances during the test period, the program assigned a repairman to work at his ex-wife's house!
Even if we had thought about this bizarre possibility, we would probably have dismissed it as ludicrously unlikely. What are the odds that the program would assign someone to work at his ex-wife's house? Murphy's Law says this had to happen eventually. The amazing thing is that it happened not once but twice during the test period.
Fortunately, this application was designed to keep human beings in the loop. The repairmen recognized their former spouses' addresses and called the dispatch center to ask for different assignments before things got embarrassing.
Nothing flushes bugs out faster than putting a system into production.