For example, AR glasses could stop you from crossing the street when there is an oncoming car, regardless of what the traffic lights indicate.
As another example, AR glasses could stop you from walking alone at night or passing through a high-crime area.
Not sure this is the best example: Maybe turning off the AR and using your eyes and ears would work better.
Killer app for me is having the manual about an item I use embedded in a qrcode on the item itself, instead of in a paper book or online.
(Not anti-AR by any means - have been working with it/thinking about it since ~1998)
Oh dear, this is going to be controversial.