When I worked for a Campy distributor I used to make a daily pilgrimage to see all the returns. Crank failures were the top reason for returns, but nearly all the problems were preventable.
The most common failure was a crack at the spider and as Steven suggests, simply rounding the sharp edge will go a long way to reducing the stress riser.
The other common failures were breaking at the pedal eye, failure of the arm due to prolonged rubbing against a front deraileur cage, catching a pedal in a corner and starting a fracture and finally splitting at the spindle hole.
All of these can be prevented by both good mechanical practices and frequent inspection. Most of these cracks are there for quite a while before the catastrophic failure.
The best place to see these failures is Ric Hjertbergs page
http://pardo.net/
Bryant Bainbridge
Portland, Orygun
> From: marcus.e.helman@gm.com
> Date: Mon, 23 Jun 2003 11:11:38 -0400
> To: classicrendezvous@bikelist.org
> Subject: [CR]Campagnolo crank breakage?
>
> At the risk of sounding like that Gilda Radnor character on SNL, what's all
> this I hear about Campagnolo cranks being subject to sudden catastrophic
> failure? Is it true? Do I really need to closely inspect my cranks before
> each ride?
>
> Thanks,
> Marcus Helman
> Huntington Woods, MI