One problem related to transmission failure has been reported for the 2006 Mercedes Benz SL500. The most recently reported issues are listed below.
722. 9 7-speed transmission has known defect causing vehicle to lock up in gear when driving. Locks into 3rd gear, can happen when you go to pass causing the vehicle to have no power when you try to kick into passing gear. On many occasions I've almost been hit because transmission fails to shift into passing gear. Also will happen when coming up to a red light then, light changes to green and you press accelerator to go but car will hardly go. I was bumped in the rear bumper once, luckily not causing any damage to vehicle. I've also been in many close call accidents when its happens. Mercedes knows about the problem, vehicle was taken to mercedes-benz of bloomfield hills, service advisor informed me its a very common issue and repair is $2100 to replace the valve body and shift conductor plate in transmission.