A problem with some manual transmission cars is that they can pop out of gear. The vehicle will simply drop into neutral unexpectedly, causing the engine to rev without actually turning your wheels at all.

Mounting

Over time, transmission mounts can become worn. This makes the entire transmission vibrate as the car moves over bumps and uneven surfaces, which can disconnect the engine from the transmission, thus putting you out of gear. The way to fix this is to replace the mounting.