There are two ways in which a machine, or any kind of causal set-up, can fail. One would be if a part failed. Suppose a vital cog falls out of the machine, for instance, leaving a gap in the mechanism. The chain of causes that travels through the machine fails at that point. This is an instance of what we could call subtractive interference, which is the taking away of something from the cause, which prevents it having its usual effect.
