I knocked on a closed bathroom door and no one responded. But, when I opened the door, someone was using the bathroom. 
I can't believe I walked in on someone using the restroom. 
user
Oh my, that must've been an embarrassing situation!
assistant
It was! I knocked first and no one answered, so I thought it was okay to go in. 
user
That's ok, you did the right thing. It probably happens to lots of people.
assistant
Thanks, I'm sure it does haha
user