This is, sadly, on a bike that I'm considering. The current owner says he bought it this way and that it hasn't changed in the time he's had it, and that it doesn't affect the integrity of the frame, as far as he can tell. I'm mystified as to what would make a lug *crack*, and if it did, why the whole thing wouldn't fall apart.

Yep, it's cracked.
Honestly, I don't think it would cause much trouble. You can always get the lug replaced if you really want to.

Oh, as for why.... I'm guessing over-zealous headset cup installation.

Be aware that the bike is not a Primato, as advertised, it's a Professional, and that he works at a local bike co-op.

FWIW, I passed on it last week.

Good Luck,

OFG

sure looks cracked, and "it happens" but the brazing may be enough to hold it all in place, even in broken condition. whether it's safe or not: I can't tell you for certain but I'd guess it's not a "clear and present danger", but still get it checked by a real frame builder before you buy.

Thanks to all, I appreciate the responses.