[camtarn]

In the case, this could possibly have been limp-home mode. I believe this engages if the ECU thinks the throttle pedal is faulty. It disables the throttle and applies a constant throttle setting to allow you to continue driving slowly - it sounds alarming in neutral, but once you put the car in gear, the revs drop as the engine is required to produce torque.

Happened to my 2001 Clio several years back, although in my case it was when I started the car, not during driving. My hypothesis was that it triggered due to my bad habit of turning the car off in traffic queues then restarting it with the pedal held down.