Review the following thread-safe observer pattern snippet. Identify the line that causes unsubscribed observers to still receive event notifications.

Review the following thread-safe observer pattern snippet. Identify the line that causes unsubscribed observers to still receive event notifications.

import threading

class Subject:
    def __init__(self):
        self.lock = threading.Lock()
        self.observers = {}

    def subscribe(self, observer, event):
        with self.lock:
            self.observers.setdefault(event, set()).add(observer)

    def unsubscribe(self, observer, event):
        with self.lock:
            self.observers[event].remove(observer)

    def notify(self, event, message):
        with self.lock:
            observers = self.observers.get(event, set())
        for obs in observers:
            obs.update(event, message)