Why having a backup isn't the same as actually surviving a failure.
Bolt a second sensor, motor, or controller onto a system and it's tempting to call the system "fault-tolerant." It usually isn't. Redundancy just means duplicate hardware exists — two encoders where one would do, a spare motor, a backup controller sitting on standby. Fault toleranceis a behavior: the system keeps operating correctly through a failure. Redundancy is an ingredient. Fault tolerance is what happens when that ingredient is combined with two things redundancy alone never provides — a way to notice a component has actually failed, and logic that automatically switches operation over to the one that hasn't. Skip either of those and the spare part is just unused inventory sitting next to a single point of failure.