Apparently stuff happens even to E320 CDIs.
Stuck Glow Plug Removal - Mercedes-Benz Forum

Google search =Mercedes OM648 with broken glow plugs

http://www.dailymotion.com/video/x27y6wq_mercedes-cdi-e320-diesel-seized-or-stripped-glow-plug-problem_auto
"Cylinder 5 scared me....."
http://mbworld.org/forums/diesel-forum/466006-om648-glow-plugs.html

http://mbworld.org/forums/diesel-forum/466006-om648-glow-plugs.html

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84 300D, 82 Volvo 244Gl Diesel