Do not feel bad about not noticing the cracks; I do not know how long I rode my Peugeot UO-8 with a cracked chainstay. [Steel cracks, too!] I had removed the rear wheel and was cleaning and preparing the BB for mudguard installation when I noticed what at first appeared to be a scratch on the stay. By the way, I bought my high-end (Tange Prestige II CrMo) 1988 Schwinn mountain bike in 1993 from the first owner, for whom Schwinn had previously replaced the 4-year-old original frame under warranty.
I wonder whether your head tube cracks could have been caused by an improperly-installed headset. For example, if the ends of the tube were not dressed to be precisely perpendicular to its axis, then some of the compressive force on the bearings would be directed fore-and-aft, leading to the eventual deformation or splitting of the tube.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069