Either when you were a noob or had no idea it was good.
My biggest regret is praetorian foil..
Edit 1: with almost 100 replies, the biggest regrets seem to be LDR and the longbow. Thank God I have mine still.
Edit 2: almost 250 replies, I didn't think this would blow up! It's still all exotics or vanilla guns that had good perks. I feel your pain guardians
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Black hammer. Dismantled maxed one on accident.