Here’s the situation: You’re about to leave a public restroom, and you open the door just as someone else is about to enter. That happens to me from time to time, and when it does, I inevitably utter, in a high pitch, the syllable “ooop!”
To me, the “ooop!” serves as an acknowledgement that I see this person, and I feel that our almost collision is an unfortunate circumstance. The “ooop!” is not meant as an apology, but if the person believes I am at fault in this mishap, I don’t mind if they think I am apologizing.
Variations of this utterance are “whoops!”, “whoa!”, and “eep!”.