just because you were right there by the car doesn't mean the car didn't have some sort of electrics glitch. back in the 90's when we were using AM radio's on nitro cars weird glitches were common place. my nitro RS4 2 took off on me on numerous occasions. i had to walk all the way across an elementary school playground to go get it. the newer 2.4 gig radios are much better but i know guys who have suffered minor glitches with those (early spectrum radios every time. mostly seemed to be a range issue).
a fail safe goes inline between the throttle/brake servo and the receiver. and you have to program it (super easy to do) to apply the brakes when the radio signal is lost or receiver pack voltage drops to low (will also keep you from over draining a lipo receiver pack). a properly programmed fail safe will bring a moving car to a dead stop by applying the brakes and hold a stationary car in place until radio contact is regained or you change/charge the receiver pack.
they're your best bet for keeping a nitro under control should something fail and there only $20.