1. A philosophic point of view. When a system fails, how should it leave things: secure or unsecure? For example, if a firewall crashes, should it disable all network connectivity, or should it allow network connectivity to continue unprotected? A lot of security vulnerabilities occur because designers make the wrong choice. It is often easier to cause a system to fail than to break through it, so security items should probably fail in such a way to result in greater security at the expensive of stopping everything. Confusion: The terms "fail-open" and "fail-close" are frequently used to mean the opposite of each other. Some people think of a door, which when "open" allows things to pass through. Other people think of an electrical circuit, when "open" stops the flow of current (and conversely, a "closed" circuit passes current). Therefore, use the word "fail-safe" instead in order to avoid confusion. Analogy: The electrical circuit-breakers in your home are fail-safe switches using this concept. In the case of an electrical fault causing a short, the circuit breaker will blow open, halting the flow of electricity. This prevents a fire from starting. From Hacking-Lexicon