My shoe lace is undone. I know it.
Two strangers: ‘Hey, your laces are undone’.
The zipper pocket of my backpack is undone. I don’t know it.
Fellow walker. ‘Hey, your back zipper is undone’ .
Stroller getting off the bus. Big helpful commotion.
Passenger: ‘Driver, there’s a stroller getting off at the next stop’.