Overheating is probably the most common cause of blown gaskets, but they can fail randomly from age or usage too. Gasket failure isn't that common on the 61x's but it happens. Head gasket failure is probably more common on the 60x series engines since they use an aluminum head on iron block (causes scrubbing) and have a stupidly designed exposed oil gallery by Cylinder 1 in early head revisions.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)