The worst that ever happened to me was the year my recipient asked for fluffy holiday het in a slash-dominated, not-Yuletide-popular fandom, I sweated bullets making that damn fic work, and then the recip disappeared into the ether. They defaulted, didn't comment, and gave no evidence anywhere that they'd even read it. I've had a lot of fics get disappointing receptions, but I've never had a recip flake on me before or after.

The worst I've ever done was probably write a tiny, high-concept joke fic for my recip in addition to the regular gift. They were confused and unamused, and I felt like a pill.