Going over large bumps in convertible models may cause the roll bar to deploy, rendering the roof inoperative. Special tools are generally required to return the roll bar to its stowed position.Google+
Car Problem Reports
Mercedes-Benz CLK550 Going Over Bumps in Convertibles May Cause Roll Bar to Deploy
Mercedes-Benz CLK550 Problem
Visitor, 2007 Mercedes-Benz CLK550
Mine is a 2005.
My local gas station mechanic can fix this... especially when the top is already down. He simply inserts a long screw driver next to the rear headrests and hits a release that lets each headrest go back down, ready for the next time.