Frames fail. I've seen a variety of frames fail, including Cannondale aluminum, Trek OCLV, Kestrel (carbon), Giant (alum), Specialized M2, Colnago ti.
The Cannondales that I've seen fail were the pre-CAAD ones, and most of them failed in the right chainstay down where they're indented for rear tire clearance. Previously I've mentioned the SuperSix with the cracked dropout which was considered to be "non-warranty". At least one of those pre-CAAD frames failed under me, but I was crashing all over the place so it's hard to say what caused the failures. I tossed a few frames that I'd bent up too much to ride.
OCLVs usually failed on the outside of the right chainstay, hence the metal chain "guard" there. I believe this was due to people dropping their chain into the frame.
Kestrel usually failed under the front derailleur mount area. I can't recall a chainstay. This is before this generation of design.
Along those lines I've seen the Aegis crack too - stays too. This is current, i.e. last year. The frames in question were ridden hard by a woman who weighed maybe 90-100 lbs, so I can't imagine it saw tons of abuse.
Giant TCR right stay, aluminum. This is maybe 2 generations ago. Since I rode this one, and I know I never crashed it, it was unexpected.
I also cracked a Specialized M2, but since I had it for a long time and it was made with "rigid" alum (i.e. probably more likely to crack), it surprised me but not really.
Colnagos had a lot of teething problems way back when. Although nowadays they seem okay, one team sponsored by what amounted to be the Colnago rep ended up breaking half their frames in a month or so - by March of the racing year most of them were on their second frame. This was the very cool looking bititan twin downtube ti frame.
I've seen a few bond failures in lug/glue carbon frames, back in the 90s. This included the Cadex, the Specialized Allez carbon, Vitus carbon.
Any frames from 10-15 years ago that you see now were probably at the top end of the bell curve for durability, ownership load (stress, maintenance, climate, etc), not crashed too hard, etc. Just like if you saw a 1993 car right now, you'd know that it made it through a lot to get to where it is now, and a lot of its brethren aren't around any more.
cdr