I had the exact same thing happen in my 1995 E420. Hit a major pothole on the side of the road passing a car, and hear a loud thunk. Didn't think anything of it, but the car was very bouncy. When pulling into my garage, the rear made a loud thunk, so I inspected the right rear. I don't know what made me look in the trunk, but when I did, the top of the shock (chrome rod) was sticking up through the carpet. Not sure why it happened, but my mechanic replaced the mount and the shock and all has been well for over a year now. Glad to know I'm not the only one.

BTW - the affected area had no rust.

Troy
1995 E420 92k