Etiquette

The other day I was at work using the restroom. I think for the purposes of this story it's important that you know the bathroom there is a one-man show and I was doing a #2. (Don't think about it too much, it's only background information). Anyways, someone knocked on the door and I froze for a second, unsure how to respond.
Now normally when somebody knocks on a door it's because they want you to open it, but the bathroom is a little different. The knock represents a question, "Is anybody in there?" As soon as I remembered this little nuance I responded to the knock by saying, "Yep," and the person went away with no harm done. But still, there's the auto-pilot effect where we don't realize what we're doing and this can lead to some delicate situations. I now have the fear that someday I'll be doing my business and there'll be a knock at the door. My auto-pilot will kick in and I'll say, "Come in," without really thinking about it. The person on the other side, also in distracted auto-pilot mode, will hear the invitation, and without thinking will open the door and walk in. If I was an artist I'd paint a picture of such a moment and entitle it, "And There We Shall Be."