These things are why I always, always, always do at least one walk around before pulling away, even from a gas pump. I check things I "know" to be good a second time. It's the: "I thought the gun wasn't loaded" theory. Also, I learned from an old timer that if I'm hooking up the toad and get distracted by anything to go back to the beginning and at least mentally, start over. Missing one step will have bad consequences. I had a buddy that let his son connect the toad to his RV. When he arrived home..no toad. The toad came lose out on the road and lucky for him went out into the trees and no one got hurt. Lesson, trust no one. Always verify and if you can't remember, check again.
My habit is start the RV, get ready to pull off, then get out and walk around, open the door on the toad and confirm brake off, key in position II, gear selector on "N", wheel free to turn. Then I reinspect the tow bar. Ready to drive. Yea, I'm anal about it.