The most harmful error of any program will not be discovered until the program has been in production for at least six months. A Batch Stream that can not be arranged in improper order will be. Interchangeable Tapes don't. Profanity is the one language that all programmers know the syntax of. Gilb's Laws of Unreliability Computers are unreliable.
Humans are worse.
Any system which depends on human reliability is unreliable. Undetectable error are infinite in variety. Detectable errors do not exist, unless deadline is less than three hours away. Investment in reliability will increase until it exceeds the probable cost of errors, or until someone insists on getting some real work done. Any manpower added to a late project makes it later.