The advantage of being a hoarder: this has never happened to me.
I keep every legendary that might even be remotely useful. For example, I crafted 30 Penders Purchase - all turned out to be crap - but I kept some of them in my stash. Later on I figured I could use one of them for my pure tank monk.
A friend salvaged a shard of hate as he "thought it was rubbish" He has a reputation for being lucky and getting the best drops so when he looted it everyone on ts started laughing at his unbelievable luck. He thought he was being trolled and salvaged it without asking.
I sharded a Starmetal Kukri very early after release. Don't remember the stats on it, but it wasn't good enough to use over what I had at the time and certainly didn't have a socket.
The Soul it gave me probably went into a pair of bracers I enchanted 13 times to get Poison damage on it and wore about 1 day after I finally managed it. It was the first time I decided I was not going to shard stuff again cause I needed Souls to make a passable item maybe decent, and there have been many times since.
A half-crappy Odyn Son i got early. Before SoH on my barb. Once i got the SoH i realized what i had done (Got a new odyn a few days later that was actually really good)
An SoJ with the wrong element and no CC or CHD (Yesterday i got one with 50% CHD and 30% elite damage though. Jackpot!) Lut Socks. I wish i had saved those for the day i get my third 'of the Earth' piece Trag'Oul Coils. I later realized those were amazing to have with you to cut of almost a minute from your major cooldowns every time you find a health well.
A shitty Sun Keeper i wished i had saved for the elite damage and some testing i could've done. Thorns gloves. (of the invoker or whatever). Would've given me 4-set, which i believe could be pretty sweet combined with the Blackthorne 4-set.
I am sure i've made more terrible mistakes than this, and if i remember i will be sure to let you know!
An Eye of the Storm with (iirc) 23% Lightning damage, 4.5crit, 700 dex and a socket two days after my Monk dinged 70 b/c I did not realize the value of elemental damage; the new Leoric's Crown I got was green on all three stats!
I have kind-of a lame one, I salvaged a Skywarden (random Law every 60 seconds) because it rolled with no STR and no socket, without realizing I could've just rolled INT on it and given it to my Enchantress. I know it's more a novelty than a real loss, but I would've liked it for sure.