If you're referring to nipple breakage when spokes do not reach and engage the heads, it's anything but an urban myth. It's a well documented fact, and happens to brass as well as aluminum nipples.

There is some debate about the mechanism of failure, whether it's classic notch failure or whether corrosion might play a role, but with the exception of steel nipples, nipples can and do fail if the spokes don't engage the heads properly -- ie. 2mm or so. Nipple failure seems to be more likely with 14g plain gauge spokes vs. butted or15 spokes, so those building with 15g butted spokes are least likely to need worry about it.

To be clear, I don't say that nipples will fail, just that they may, and the risk increases with time and wheel stress. OTOH, since failure an be just about 100% prevented with proper length spokes, it's hard to justify short spoking a wheel you spend serious time and dollars building in the first place.