"Things that are impossible just take longer." - Ian Hickson
Software systems are like machines with small cogs and gears. If any of the individual gears malfunctions, the machine as a whole is most likely to behave in an unreliable manner. In software, the individual gears are functions, modules, or any libraries that you use. Functional testing of the individual components of a software system is an effective and practical way of maintaining high quality code. It doesn't prove that bugs don't exist, but it helps in building confidence when deploying the code to production and maintaining the sanity of the code base when the project is to be maintained...
