For example, a rocket engine which can use both liquid methane and liquid hydrogen.
The RL10 has been experimentally fired on methane and propane as well as hydrogen. This did entail modifications to several components of the engine; for the methane version:
Fuel component modifications were unique and were accomplished under the contract; assemblies that were changed were the pump, turbine, thrust control, injector, and thrust chamber.
So it’s not exactly the same engine.