If you improve or tinker with something long enough, eventually it will break or malfunction.
An expert is the one who predicts the job will take the longest and cost the most.
Investment in reliability will increase until it exceeds the probable cause of errors, or until someone insists on getting some useful work done.
Any given program will expand to fill all available memory.
Friends come and go but enemies accumulate.
The Ninety-Ninety Rule of Project Schedules - the first ninety percent of the task takes ninety percent of the time, and the last ten percent takes the other ninety percent.