db.somecollection.remove({_id : undefined}); removes every item from the collection. I think that the correct behavior would be to throw an error.

Example:
db.temp.insert(

{ _id : 1}

);
db.temp.insert(

{ _id : 2}

);
db.temp.remove({_id : undefined});
db.temp.find(); // returns nothing

Same thing happens with any undefined property:

db.temp.insert(

{ _id : 1}

);
db.temp.insert(

{ _id : 2}

);
db.temp.remove(

{foo : undefined}

);
db.temp.find(); // returns nothing

I could easily imagine a situation where a code like this: db.users.remove({_id : user.id}); would be called with an user object which doesn't have id set, which would end up calling db.users.remove({_id : undefined}); and thus removing all users from the entire database.

I know that the programmer should have prevented this error, but I also believe that the database should be smart enough to prevent such disaster. After all, the programmer should be using .remove() without any additional parameters or just .drop() if he wants to erase the entire collection.