Interchangeable Tapes don't. Profanity is the one language that all programmers know the syntax of. 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. Fuzzy project objectives are used to avoid the embarrassment of estimating the corresponding costs. Carelessly planned projects take three times longer to complete than expected.