I've been following this thread and I feel it is time I voice an opinion
Hypothetical scenario: Person A really needs to take a crap, so they go to the restroom in their house, closing the door but not locking it. Person B, who lives in the same house, also needs to take a crap, so they try to enter the bathroom without knocking. A terribly awkward moment results. If you could place the entire blame for this on one of them, would be Person A at fault for not locking the door, or is Person B at fault for not knocking?
From the OP I make several assumptions.
1. There is a lock on said bathroomdoor
2. It is a working lock
3. The custom of the house is to lock the door when you're using the bathroom
4. Person A and Person B both know about the customs of the house.
In this situation it is only logical that Person B doesn't knock. When he came at the door he opened the door because he wanted to use the bathroom.
Let's assume that said lock shows whether or not it is locked or open. In this case it would have shown open and therefor meaning that the bathroom was free to use. It is only logical that Person B opens the door in this case.
Let's assume the lock doesn't show whether or not is is locked or open. If this is the case the only way to know if the bathroom is free is to try the door. So Person B tries the door and it opens and he/she walks in.
In both this cases Person A is to blame because it was him/her who failed to meet the custom of the house.
Off course Person B can appologize out of courtesy but he/she isn't to blame.
However, people in this thread have stated that it is custom in there houses to knock on the bathroomdoor before entering.
If this would be the case in the situation of the OP than Person B is obviously to blame because he/she failed to meet the custom of the house.
I would like to make a sidenote to this. If a bathroomdoor has a lock you should lock it if you want to be 100% sure to have privacy even if it is custom to knock. You can't fully blame the other person when you have the possibility to lock the door.