A beautiful artisanal custom bike I saw recently had a fatigue-cracked head tube, right at the downtube lug edge, and it was not nearly the first one I've seen.
The cracked frame had lugs filed down so small that they fell into the category of "bikini" lugs. (The name doesn't refer to "two-piece", as with the swimsuit, just the tinyness of it. A lug that doesn't cover up much of the joint.) That's not a bad thing in itself - I like bikini lugs.
In my experience, lugs with a too-small "hand-full" of head tube will sometimes (depressingly often in fact) cause a crack in the head tube at the DT lug, for riders over a certain size/weight and/or miles/power combo. Cracked head tubes are relatively rare for frames with a little more meat there in the DT lug - including cheap frames.
An example: Of the two ultimate (to me) '80s-vintage investment cast bikini lugs, Ohtsuya and Eisho, the Ohtsuya will crack head tubes at the DT for big/strong/hi-mileage riders, where the Eisho won't. The difference is subtle, but enough.
Eisho is probably best known from Nagasawa:
http://bulgier.net/
I don't have a great photo of the Ohtsuya DT lug but I think you can see
the problem area here:
http://bulgier.net/
For years (late-80s - early 90s maybe?), Richard Sachs's ad in the bike magazines was a picture of someone's hands holding a file poised over a lug. (Presumably the hands are e-Richie's - though he wasn't known by that name yet!) The lug in the ad is an Ohtsuya DT, which is pretty funny in itself, since there is NOTHING to file on those lugs, but especially funny because the file in the photo is shown poised to file RIGHT on that one edge on that lug that is already way too small - at the red arrow in my photo. I believe this was an in-joke from Richie to the few people who knew how ridiculous it was to file right there, on that lug.
Just so we're clear - this is NOT a slam on Richard Sachs, who has designed lots of lugs, all of which have plenty of meat at that point I'm talking about. All the more reason I think the photo in his old ad is an in-joke.
The moral is, lugs can safely be carved down pretty darn small, but there is a minimum, which varies from one area to another depending on the stresses on that area. And the headtube edge of the downtube lug is one area where some lugs are carved down too small - fairly frequently actually, in my experience, at least among high-end or custom builders. (Cheap frames break too of course, but for different reasons and mostly in different places.)
Mark Bulgier
Seattle WA USA