I must be quite the FOTM sinner here:
Little Dot MKV: Didn't go well with my K701s at the time so was a poor choice.
Zero DAC: Flavour of the year, not just month. Quite musical, easily upgraded for better SQ, and I learned from it, but assembly was a bit iffy. No regrets.
Compass: Yeah, you can blame me for that. I was honestly trying to fix the drama over the Zero and dud units/peoples' headphones being destroyed and whatnot and it became a drama in itself.
Lavry DA-10: Not quite FOTM, but close, when along with the Benchmark DAC 1 it was one of THE DACs to own. Too digital-sounding for me and needs dedicated RCA output, but is still a good unit.
Northstar M192 MK1: Not quite FOTM either, but still had a burst of popularity, as it was selling for <$1k when it was $2.5k new, and had talk of having an analogue-like sound which I think gave us glazed eyes. Not quite as natural-sounding as claimed and only marginally better than the Lavry.
HD-800: It's all Jude's fault.

I don't regret them as I can still put them on and not feel I'm missing anything (except a nice tube amp to power them).
HF-2: Just put them on again now, as they haven't had any playing time recently. Still quite fun, but so are my Stax, which are more resolving. I don't regret buying them because I knew at least I can get my money back in the future if I they don't work out.
HE-5: I'm glad I had the chance to experience orthos of some description. Neither happy nor regret buying them, I just have too many really good headphones I enjoy listening with now.