I had a Mercedes SLK 350 about ~5 years ago (leased). I was at a stop light in the Chicago loop, and while holding the brake, the engine revved up 2-3K RPMs with no accelerator pedal input. After taking it in for work, I was told it was a software issue and had been resolved.
Hello, I signed up specifically to argue with you (be proud).
I think that may have been an overzealous algorithm giving the car a blip of throttle to stop it from stalling when it is idling.
An engine still needs fuel and air to run when even when there is no load on it and this is controlled by a computer. This computer targets some rpm for idling. If the rpm drops substantially low, due to for example a 'mechanical' glitch like dodgy fuel, then the computer may over compensate with too much throttle, causing the revving you experienced.
I'll double on this. I had an SLK230 ('03) and noticed that when at a stop light it would sometimes rev up to about 2k rpms and come back down. I started watching for it and noticed that it happened right after the rpm needle dropped into the 500 range so I think it was an anti-stall feature in the car.
a) Brake pedal is fully pressed b) Vehicle speed is at zero
If these conditions are met, and you're attempting to prevent a stall, you may want to consider pushing the transmission into neutral to prevent "unintended acceleration". Transmission is drive-by-wire, so could be done by issuing a command over CANBUS.
According to the comments on this thread, that scenario can apply to ANY manufacturer. These problems are inherent to the technology being used, not the users of the technology.
I don't lease/purchase Mercedes vehicles anymore.