What causes an `AttributeError` when using a dictionary?

An `AttributeError` with dictionaries happens when you try to access a key that doesn't exist. For instance, `my_dict['nonexistent_key']` will raise an `AttributeError`. To avoid this, use `my_dict.get('nonexistent_key')` which returns `None` if the key isn't found, or check if the key exists with `if 'nonexistent_key' in my_dict:`.