interesting thread, i'll contribute.
worst i've heard and paid for: koss plugs (hahaha) after i had shredded my portable er6i set. i was traveling and desperate, i'll never do that again.
worst disappointment I've paid for: pioneer steel wheels se-mj71 closed headphones from the amazon sale earlier this year. bass rolled off in a bad way from a massive bass hump that bled a truly annoying thudding noise all the way into the mid-range, so having said that there was very little mid-range. there were no highs, just an immense shrill that then overpowered the bloated bass from the other end. a V headphone this wasn't, more like a very skinny U. the soundstage was small, as if all instruments were positioned in one spot, about 8 inches from your face. I was rightly offended. after an extensive burn-in process very little changed and i gave them away. didnt feel right selling them. anyway ...