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"Fail safe" means: it won't fail.
"Fail fast" means: it may fail
... and the failure condition is checked aggressively so that the failure condition is detected before damage can be done. The alternative to "fail safe" and "fail fast" is to fail unpredictably; e.g. to sometimes give the wrong answer or throw some unexpected exception. (This was the behaviour of some standard implementations of the Enumeration API in early versions of Java.)