What might be a consequence of over-revving the engine in a lower gear?

Over-revving the engine in a lower gear can lead to engine damage due to excessive revolutions per minute (RPMs). When an engine is operated at higher than recommended RPMs, it puts significant stress on the internal components, such as the pistons, connecting rods, and valve train. This can cause severe mechanical failures, including bent or broken valves, damaged piston rings, or even catastrophic engine failure if the engine's redline (the maximum safe RPM) is exceeded.

The other options do not apply in this context. While improved acceleration may seem possible due to the engine being in a lower gear, it is not a sustainable outcome, as the risks of over-revving outweigh any temporary acceleration benefit. Increased fuel efficiency is generally the result of optimal RPM maintenance, which is contrary to what happens during over-revving. Lastly, over-revving can lead to rough engine performance rather than smoother performance, as the engine components could be under duress.