Of the “I bought this and it was bad” variety: Batman Rebirth volume 4 (The “War of Jokes and Riddles” storyline). I was already sick to death of King’s writing at the end of Volume 3. I knew I wasn’t going to like the next volume, but it had the Riddler in it, so I thought I’d give it a chance. It read like a bad fanfiction and at that point I was done, wedding or no wedding. Turned out to be a good time to jump ship.
Of the “I wasted my money” variety: The first volume of the collected edition of Knightfall. I bought it like three weeks before DCU uploaded the entire story arc. Whoops!