Our wakeup call as newbies was when I hadn't yet chocked the wheels and Jack, not knowing the trailer wasn't chocked, removed the chains. Our campsite was in a small campground up on a hill and the back of the site had a wooden safety barrier because it dropped off to the road far, far below. The site was dirt and not very level. We were using the wheel on the tongue jack. The trailer started to roll back, towards the barrier and I remember Jack grabbing the hitch trying to stop it. Of course, he wasn't able to stop it but fortunately the site was also sloped to one side, so the trailer went into soft dirt at the side which stopped it. If it had gone straight back and through the barrier, it would have fallen on the road about 40' - 50' below. I shudder to think what harm that could have done, not just to the trailer but anyone on the road below.
That's why the very first thing I do is chock both wheels, front and back. Always!
This was on our trailer when we purchased it from the previous owner. In our inquiry about the signage, he described seeing a trailer going through a campground by itself while the owner stood beside his truck in horrifying amazement.