I had that happen on a little Saab 340 out of LAX to SAN. We taxied normally, ran up the engines, got up to speed rather quickly and just as you would have thought we were about to leave the ground..... airbrakes, reverse thrust and heavy breaking. It is indeed quite unsettling. To be fair, with a Saab 340 on LAX's runway, we still probably were not even half way down the runway when we finally got turned off onto the taxiway again so we were probably a good mile from overrunning the end of the runway, but still kind of disturbing. My real first thought was a mechanical and we would probably be taxiing back to the terminal for a nasty delay that I really did not feel like dealing with at that point having just spent the last 20 hours in airplanes and being a half hour flight from home. Not a word out of the cockpit as we taxied, but instead of back to the terminal, the engines kind of ran up and down a few times and then we taxied back to the start of the runway, turned back on and took off perfectly normally. So then I figured if the aircraft was fine, it must have been some kind of runway incursion, which is even scarier. Only after we had finally climbed to cruising altitude and leveled off did the pilot eventually come on and said something like "sorry about that takeoff, we had a bad oil pressure reading but it turned out to be fine after all." I suspect everyone on the plane would have appreciated a more prompt explanation than was given.