Mine was dismantling Helm of Saint-14 when I was a noob...
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I accidentally got rid of the only Truth I bought from Xur. Haven't got one since. It's pretty decent in PVP, and I didn't realize it until after the fact. Felt like a n00b. That's probably it, though. Actually, I forgot to upgrade my remaining two Gjallarhorns from 1.0 exotic stats to 2.0. Laziness, more than anything.