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I'm looking for a word (or phrase) to describe mechanisms that are perfectly functional even when they aren't functioning as expected.

Examples of these include:

• 
  Escalators & Electric Walkways: Even when they aren't powered these systems still function as perfectly adequate stairs or sidewalks


• 
  Mopeds: Even when out of gas, the owner can still pedal along


• 
  Electric Toothbrushes: One can still brush their teeth without turning it on

I've considered using engineering and management terms such as redundant mechanisms, enhanced systems or fault-tolerant machines that come close but those terms refer to very specific technical concepts that don't apply to my needs or the the examples.

If it avoids catastrophic failure, it exhibits:

graceful degradation - Graceful degradation is the ability of a computer, machine, electronic system or network to maintain limited functionality even when a large portion of it has been destroyed or rendered inoperative. The purpose of graceful degradation is to prevent catastrophic failure. Ideally, even the simultaneous loss of multiple components does not cause downtime

I do not think you will find one word for these things. They are not "working" as designed... they have failed. They simply have failed in a way that doesn't completely destroy their utility, but that's not to say the couldn't fail that way. The wheels could fall off the moped, the brush on the electric toothbrush could break off.

You're more describing a failure mode than a specific variety of things. So, another answer geared at a description, rather than one word.

These designs feature inherent utility.

Their design features an intrinsic quality that makes them utile even when they're not fully functional. In fact, you could use the word intrinsic instead of inherent.

These are not standard phrases, but I think are understandable on their faces.

Also consider redundant utility.

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Additional answer:

We can also say the design of these devices has a built-in fail-safe.

Fail-Safe

1 : incorporating some feature for automatically counteracting the effect of an anticipated possible source of failure.

Alternate Mode Machines - auto to manual?

These machines are manually operable.

Assuming one mode as powered by a motor as automatic and a secondary non-powered mode of use as manual such as in the case of escalators or mopeds, all of these items have the same in common. They can be operated manually.