Never noticed on the XX78's but the X7's I used to find cracks in the shafts down near the points that ran up the shafts. Not just a discoloration on the anodize but actual cracks. I believe the 78's and X7's are made from the same aluminum but never had problems as you have stated.
I bought a bunch of raw shafts back in '01 or '02 that seemed to have a problem with cracking. Some of it was from group shooting the arrows and one shaft hitting the other causing a crease and a crack, but others had not been hit and were cracking. Most of the cracks were from the label down towards the tip. I assumed that I got a bad batch of shafts as I've bought others that I've shot for many years with no problems.
The only cracks that I seen were on my 2312's if it was hit by another arrow or if I hit something hard that was stuck inside a target like a field point or part of another arrow, but never had any crack on my 2216's or my 2117's and I have been shooting them for a long time.
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