Along the lines of what stupid thing have you done today. I know better than this.... I can't believe I forgot to close the black tank valve the last time I drained. Now I have a mess! A remin...
I feel for you. I did something similar on my trailer. I thought the black tank valve was all the way closed but it wasn't and when the cap came off it was a gusher. Now I have placed one final valve at the cap that will help to keep that from ever happening again.