Has anyone ever seen a Cannondale frame fail?

Yes, I've seen many fail. I use to do all the warranty processing for a large retail bicycle chain of stores that sold Cannondale's. They don't have any higher of a failure rating than any of the other brands of bicycles except on the very high end of the line. The higher end bicycles are "cutting edge" and often very high tech. Since the bicycle manufacturers try to make their bikes as light as possible it's expected to have a few more frames fail. Aluminum frames typically have a shorter average life (15-20 years) than Cro-Mo or Carbon.

Aluminum does fatigue, I'll give you that. Though the mast on my 35 year old sailboat is aluminum, and I don't think it's going to fail this year or next. Imagine the stresses that thing endures on a 27 foot 8000 pound boat. I've seen a few aluminum masts fail, and it's almost always attributed to the failure of a stainless steel shroud (the stays that keep the mast upright), or a stainless steel turnbuckle or other fitting.

But my real question is this: How many carbon frames have been around for the 15-20 years it would take to know that they've got a longer average life than aluminum? ...just wondering.

i've seen plenty of trek oclv and giant cadex frames out and about.

My Giant CFR1 (carbon tubing) which is basically the same as the Cadex botto mentioned, was going great until I got hit by a motor bike last year. Even then the carbon appeared unaffected, but the alluminium crown cracked. It was purchased in 1995.
I've seen two rear dropouts snapped on older Cannondales (maybe 16 year old frames). To be honest I don't know the background of their failures, but I'd guess it was due to an accident.

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

I broke a C'dale MTB frame many years ago. It was back in the crack and fail days and I got an immediate replacement.

I have an F400(I think) from, I want to say, 1996. It's been beaten around quite a bit in the "wilderness" and has been wrecked a bit. I've yet to see any cracks develop or anything to that extent.

I have a 98 CAAD3 that is still going strong. I'm 220-245 and still use it as my organized ride bike on centuries. Some with 10,-12,000 ft of climbing. That's plenty of stress from a big body and it's still great!

Some buds have told me years ago to watch it as it may burst into flames. Hasn't yet but plenty of their bikes have. Ti, carbon, steel