According to BBC right-wing extremists that attended “Rock für Deutschland” was given free t-shirts with the following print (“Hardcore Rebels”):

However, when they was washed the hidden message was reviled:

The new message reads: "What happened to your shirt can happen to you. We can help you break with right-wing extremism".

The prank (I’d call it a cool hack) was done by the group Exit, who is trying to help people who has not been totally brain-washed to change their ways and turn their back on neo-Nazism for a more balanced world view.

Really cool hack, I wonder if it could be used for some sort of prank at next years Defcon?