A 911 operator did a double take when he heard a "Fear Factor" stuntman fell "a couple stories" ... leaving the man with two broken ankles.

TMZ obtained the 911 call made by someone on the "Fear Factor" production team immediately after the accident ... which TMZ first reported last month.

During the call, you can hear the operator's shock when the FF staffer tells him how far the stuntman fell -- which, by the way, was an undersell.

The guy actually fell SIX stories ... when his safety harness didn't function properly.

TMZ has obtained video of the massive collision that took place on the set of "Fear Factor" recently -- sending one contestant to the hospital -- but it could have been a lot worse ... if the ambulance hadn't arrived in record time.

As we previously reported, one of the contestants on the show was strapped to a truck for some stunt -- but something got in the driver's eyes ... and he plowed into another car.

Thankfully the guy was going less than 20mph at the time -- and the ambulance arrived LESS THAN A MINUTE after the accident.

The contestant was taken to the hospital -- and was released hours later with no serious injuries.

A "Fear Factor" stuntman is lucky to be alive -- after a safety mechanism FAILED during a jump from 6-stories high last week and he hit the ground FEET FIRST ... breaking both of his ankles.

Fortunately, the stuntman was attached to a back-up safety mechanism that helped slow down the fall ... but sources tell us people were "very concerned" when the stuntman hit the ground.

We're told the stuntman was testing out a new challenge that will be featured on an upcoming episode of the show.

According to our sources, medics were on standby and quickly transported the stuntman to a hospital to treat his injuries.

"Fear Factor" producers tell us, "Safety is our first priority, and because of that we test these stunts repeatedly with trained stuntmen to ensure our contestants' well-being." They added that this particular stunt was part of their standard safety process, and wished the stuntman "a very speedy recovery."

This is the second accident on the set this month -- as TMZ first reported, a contestant was taken to the hospital (but suffered no major injuries) after the truck she was standing on slammed into a stunt car.

A "Fear Factor" contestant got a little more than she bargained for when the stunt truck she was riding on top of hit another car ... but she escaped with no major injuries.

We're told the driver of the stunt truck -- which was going less than 20 mph at the time -- had his vision blocked for a brief period of time, resulting in the truck hitting a parked car that was part of the stunt.

According to our sources, the woman was in a safety harness on the truck and she was shook up from the collision, but was not thrown from the vehicle. She was transported to the hospital by an on-set EMT. We're told she suffered no major injuries and was released from the hospital after a few hours.

A source close to the show tells us the episode stopped filming for the day when the accident occurred, but will resume shooting this week. The woman has not decided if she will return.