I had a runaway when I first had my Storm. The antenna wired exited the reciever box a little too close to the rear center shaft, and got wound up around it like a spool. Thankfully, it was about 10 feet away and approaching, so I was able to catch it and shut it down without damage. And this was on a new novak XXL... I was ticked, but got over it.
Another incident was the same car. Breaking in the engine (Ofna O1comp BP) in the driveway, and the switch died. The car took off to the street at about 1/4 throttle... I caught it, just before it made it to the middle of the road.
Other than that, I've been really lucky to not have any runaways. Altho, I am a firm believer in throttle return springs. My GT's linkage fell apart during practice one day (that little circle-clip came off the throttle servo horn). I went and paid the $5 or so for the Tmaxx linkage kit (it had the same part, but it has a nut on the bottom instead of a clip).. Best $5 I ever spent on a traxxas part, lol. Haven't had a problem since. If it wasn't for that little spring pulling the throttle shut, it probably would have went into the middle of a cow pasture.