QR codes are not as naive as they look and can hide explits like wipe out your smartphone data.
At Ekoparty confrence in 2012, Ravi Borgaonkar demonstrate how he wiped out Samsung S3 data
using USSD (Unstructured Supplementary Service Data) code.
From Wikipedia –
(USSD) is a protocol used by GSM cellular telephones to communicate with the service provider’s computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network
Another short video: