I was just fiddling with my B&W and it HAS to be pinned open in order to unhitch. If the kingpin is putting any forward or neutral pressure on the jaws, the handle will not stay open. So...once hitched, even if you forgot to pin it, you likely would not be able to lose the trailer. If the truck is able to roll forward, or the trailer backwards when the handle is opened, the jaws will release.
So I think my scenario with the brief un-hitch-re-hitch and forget to remove the pin is about the only way I could have screwed it up! Whew!