The ones I remember the most are from back in the 2E days. The first was a +5 Defender Two-Handed sword. Since everyone in the group was proficient/specialized with other weapons (mostly long sword), nobody wanted to take it. Both the detect magic and identify indicated how good it was, but they didn't want it in spite of that. At the time, the best weapons in the party were around +2 with other abilities. The group sold off the item and then promptly ran into some beast that needed +3 or better to hit. Served 'em right. I was a co-DM in the group, and I handed out the item, but another DM ran the group into the big beastie, so I wasn't punishing them or anything like that.
The other item I handed out that went totally unused was a special sword. It appeared cursed at first, but it was actually unbreakable and had the ability to break other items it hit (weapons, shields, armor, etc). Again, this is back in the 2E days before Sunder was handled in the rules. Anyways, the sword seemed like a cursed long sword since it always appeared in the owner's hand when he attacked, and seemed to have no attack bonus. They worked and worked to exorcise it and eventually figured out a way to get rid of it. All of this without ever bothering to identify it. I had a whole chart designed to handle it in combat, etc. After that, I gave up designing specialized items for that group since the players were just too ignorant to take advantage of them.