The whole glow plug system seems somewhat fragile. The dealer warned me about this too, he said that as soon as one plug fails, they just recommend replacing them all including the glowplug module. They did mine all under warranty last year so I'm hoping to not have an issue. I think the trick is to replace the failed glow plug as soon as it fails versus driving around longer, its almost like a serial (or is pararrel?) circuit so that if 1 plug fails, the other plugs get the extra voltage that the failed plug is not consuming and is more voltage than they are designed for.
2008 WK "S" Limited bright metallic silver exterior, light graystone interior, 5 speed W5J400 transmission, June 18/2008 build, C213RE rear differential with 3.73 axle ratio, C200FE front differential with 3.73 axle ratio, 245/60R18 Goodyear Fortera tires (stock summer tires), NV245 transfer case, GT2056V turbo, 3.0L CRD (OM642.980)