FBinNY 

Let's define terms. At proper working tension (within the elastic limit of the spoke) a spoke will elongate from tension, and return to it's original size if the tension is relieved. The most that a spoke can stretch this way staying within the elastic limit is about 1/2%.

OTOH, if you stress the spoke enough to stretch it beyond it's elastic limit, it will stretch and no longer return to it's original length. Essentially, you've cold set the spoke to a new longer length. Many builders forget to adjust the working tension to the spokes' capacity when working with light spokes, taking them beyond the elastic limit, and later coming back and re-adding more tension, repeating the process until they run out of thread.

It's fairly simple, if you permanently elongate any spoke, it's because it was stressed beyond the yield point, ie. built too tight for the strength of the spoke.

While I'm here, a bit of spoke strength math. If you reduce a spokes gauge by 25%, as in going from 2mm to 1.5mm you'll reduce it's load capacity not by the same 25%, but by 44%, since the strength is proportional to the square of the diameter. Knowing that, you can see how easy it would be to overload (and stretch) light spokes if building to the same range of tension you're used to using with 2mm spokes.