From Der Spiegel :
Pranksters in Berlin took things a step too far when they filled footballs with concrete and displayed a sign goading fans to kick them — with inevitable results.
“Can u kick it?” was written on a poster near the innocuous looking footballs. So, probably without thinking too long about it, two young guys, aged 21 and 23, took up the challenge and gave the balls a good kick. Unfortunately the leather balls had been filled with concrete and weighed around 10kg each. The men sustained foot injuries so severe that they had to go to hospital.
According to the police, the balls were tied to lampposts, banisters and trees near the Fan Mile in the city center, a spot where thousands of football fans pass through each day.
Police spokesperson HansjÃ¶rg DrÃ¤ger told Spiegel Online, “Unfortunately we have no information about the suspects.” He confirmed that they are being sought for assault. The police are still not certain if all the balls are out of circulation, and have asked Berliners to report any suspicious looking footballs immediately.