Melbourne Mayor says vandalism of QR codes for reporting graffiti 'frustrating'
abc.net.au
abc.net.au
Well, something similar already happened for parkings in Texas, and another one - seemingly - about QR swapping for rent bikes :
https://news.ycombinator.com/item?id=29814973
I think it greatly depends on how the app on the phone behaves (I am not familiar with them, but I have read reports of apps that directly access the link in the QR code and even if it asks for confirmation if the link looks legit enough it will trick many people).