Problem: Code alternation and placement
This QR code has been changed (shadow inside the code), is on an angle, as well as being put across the street. The QR code, because of the alternation and perspective, won't scan. It's also questionable with safety - people may well accidentally step into the road without paying attention. Do you want that law suit hitting your desk?

Problem: No internet access
Here, Red Bull put QR codes on a subway advert. However, there was no 3G signal to access the URL the QR code pointed to, so there no chance of anyone actually visiting the linked content. The recent subway campaign by Tesco in Korea worked because they have 3G repeaters on underground. The same could not be said for London Underground for example, so think before you QR.

Problem: QR code too large for people walking by - too much data
People walking past this code would need to stand back a good few metres to be able to scan the QR, possibly even needing to cross the road to do so. This QR code also has a large amount of data contained within it - it would have been better to use a URL shortener and then encoded. This would have allowed the code to be reduced significantly in size - less data means you can make a QR code smaller whilst still remaining scannable.