In the progression of time, a Failure Mode comes between a Cause and an Effect. One of the most confusing issues for new practitioners of FMEA is that any Cause that itself has a Cause might be a Failure Mode. Any Effect that itself has an Effect might also be a Failure Mode. In different contexts, a single event may be a Cause, an Effect, and a Failure Mode. Consider for example, a series of events that could occur during the life of a disposable penlight.
In an analysis of the exterior casing of a penlight, "Allows excess moisture" might be a Failure Mode. One of the intended functions of the penlight case is to protect the internal components from excess moisture during normal operation. A failure to prevent moisture during normal operation is a Failure Mode since protective casings and other design features are intended to prevent moisture.