Here are my fumbles (I already sold them):
Tiles: shiny and pretty but a pain to actually use them
3e Dragonlance....
2e Maztica box and The Horde box and adventures (just not my taste at that moment)
3e Hero Builder'd handbook: just where it is the content I pay for?
MERP: looked like a classical piece of history for me when I bought it.
Dragon Age dice set: the first set makes impossible to read the numbers.
3e forgotten realms books: just a lot of weird ideas all mixed together (some are good, really good, but the whole doesn't get me). Some adventures are great. I tried to embrace the setting but for every step my players make I have to do a research through books, novels, pdfs, forums, tweeters and phone calls to greenwood to know who is the NPC they just meet. That's why Im moving to tal dorei after my dark sun campaign ends.
4e core books: this edition has some good stuffs like menzoberranzan. But the system is too structured for me.
5e princess of the apocalypse, some sweet ideas here and there but a lot of things don't make sense and hurts when you try to run this campaign.
I almost forgot! The worst thing. The Randal Morn trilogy, railroad where players see the Randal's adventures, I got tired of that crap and for the last scene I closed the adventure and gave "free will to my players". They protected the city in a "kind of civil war" like the heroes they were, there were caos and mayhem and for all of us that was the (almost only) good part.