What was your first RV noob disaster (or near disaster)?

I was unhitching the trailer. When the hitch cleared the ball, the trailer rolled back with the tongue jack sliding off the blocks I'd put down. You guessed it. I had forgotten to chock the wheels.

Fortunately, nothing was damaged and I could raise the tongue high enough to go over the ball on the truck. I was able to begin anew, just remembering this time to set the chocks first.