On the flip side if it was in gear (and you didn't notice) then letting the clutch out after it's started it could make things a lot worse
This happened to me once. Couple of years ago we went to France on the ferry and I was driving. I rarely leave my car in gear unless I park on a hill, but of course on the ferry you are meant to leave it in gear as the ship is likly to roll around.
Anyway, we got back in the car. I dipped clutch, started engine, but I still hadn't checked the gearstick as I usually do. So, with engine running and car in gear, I instinctively took my foot off the clutch. Horrible noise, nasty lurch and engine stalled...luckily the handbrake was still on and I didn't hit the car in front. Much embarrasment for me though
