I haven't done anything to the fuel system besides replace filters and injector return lines, so it shouldn't be a leaky O-ring. The fact that they updated this part with a banjo bolt and fitting makes me think that this was a known issue. I'll order the parts from the stealership tomorrow and see if installing the new line fixes the leak (and associated poor fuel economy).

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RIP:
2011 E350 BlueTEC, Schwartz 81k (totaled by uninsured drunk driver)
1964 Unimog 404.1 (wrongfully towed by the city)
1994 E320, Brilliantsilber 208k (transmission shuffled off after sudden catastrophic fluid loss)
1982 300D, Silberblau under a blue repaint 256k (sold for parts)
1995 E320 Polarweiss, 131k (sold for parts)
1995 E300D Polarweiss, 287k (totaled by drunk driver)