Thought this might be a fun topic to vent. 
For me the worst was likely the DT 1990pro. I believed the reviewers that hyped this headphone a bit; I didn’t realize at the time how many people were into bright headphones [treble murder for my ears], but at least I learned to steer clear in the future. I didn’t fall for the hype the next time around for the Arya Organic, thank goodness. Because after demoing them I realized they are even brighter.
Simgot EA500 LM.
A terrible IEM with no cohesion whatsoever between the driver types. Too much bass, treble way too crispy.
An incredibly overrated IEM that I fell for and returned quicker than you can say Shenzhen.
Campfire IO, the midrange was unsalvageable. To their credit, Campfire honored the return so I lost nothing but my time and maybe a little on shipping. It did keep me from most blind buys from that point on, though.
DT 1990 and HD660S (1 and 2) are the only headphones I actually returned (out of many)
I realized I don’t like studio headphones lol so boring and lifeless I don’t even want to use them for gaming
I bought into the hype 20 odd years ago and purchased Grado SR80, 
……….Wot….The….Actual…… 
Westone UM Pro 50…
Absolutely awful tonality. It wasn’t cheap, either. So bad I wouldn’t sell it to my worst enemy.
Weirdly, Westone redeemed itself in my eyes with the wonderful MACH 70.
I have my shelf of shame as a reminder to think before you buy. The Focal Elegia is maybe the worst offender here. Not even worth the $350 I paid for it, and the Dekoni Stellia pads also did not help.
Moondrop Aria. They’re not bad per se, just bland like listening to a glass of milk. I will say, look to see if there’s a headphone bar in your area. I have quite a few if I’m willing to travel for an hour or two. You can never tell whether you’ll like a headphone based on reviews, you can’t really know whether the sound they like is comparable to yours even if they describe it.
Oooh… what are the beautiful IEM in the middle? Penon Fan 2 on the right?
Beyerdynamic T1 3rd gen which took the T1 from a brighter tonality to a bloated overly warm / dark mess, Beyerdynamic Amiron due to the fact it is the only headphone that was so bright it hurt my ears, Senneheisers EPOS headsets thinking just because its sennheiser makes them good which was a laugh in itself cause i could write a multi-page essay on the problems there
starfield was good when it came out, Aria was a budget starfield that came out later lol.
A lot of the 2018-2022 IEMs did age like milk with how quickly IEMs were changing lol.
That’s the Tangzu Nezha. Indeed it looks fantastic, but that’s where the fun stops.
I should have known something was up when they were vague about the drivertype for high freq’s.
The tuning itself is very boring. Detail is ok, but whatever they used (I think ended up being a pzt?) for those higher freq, it sounds really off, very ‘tin can’ like.
Tangzu is great for sub $100 sets. If you go above that, don’t blind buy.
EQ is the only saving grace of the Elegia, and even that isn’t the greatest fix for its weird mids.
Hand on heart I don’t think I have one, all of my purchases have led me to where I am today 
However one buy did define what type of replay wasn’t my thing…Sennheiser HD 660’s add 600/650/xx etc.
Most boring (yeah done tubes etc) musical date I’ve be on 
Oh and FiiO FH7 too many tuning options and wack timbre
I’ve kept the rest as Strartum of my audio replay appreciation, tech improvements and my love of music.
I like the Nezha. I like the Bajie as well. There is a particular “distant” sound to Tangzu’s pricier stuff.
The Night Oblivion Butastor was a bad buy for me. Bought it new with a little discount. It’s not bad but it’s not good either. Which for several hundred is not where you want to be. Equally the Moondrop Blessing 2 Dusk when it came out. Fell for the Crinacle hype on what turned out to be an okay set. Although that level of influencer promotion seems quaint by today’s standards,
The Symphonium Meteor I actively dislike but at least I got it cheap.
Worst set I’ve ever had was the Shuoer Tape Pro. Played some things great with a weird metallic edge. Everything else sounded an absolute mess.
Probably the Fatfreq Deuce. Simply because of how Fatfuck’s bullshit poisoned my enjoyment of the set. I can’t even look at the case without remembering that fiasco.
Same Here !!
DT177x GO easily. I have no idea why I did it, I wanted a closed back for gaming but I knew better than to just jump on something like the 177x GO but they went on sale the day I got curious and just grabbed one. Still to this day after all the headphones I’ve sold, I keep it to remind me to think about what I’m doing and I still put them on every once in a while just to hear how shitty they are. They’re not the worst headphone I’ve ever heard, but they’re damn close and they’re a perfect example of a headphone that just can’t be fixed with EQ whatsoever. No redeeming qualities other than decent build quality. With some pretty serious mods to them, they could kinda get some clarity back, but there is just something awful about that generation of driver that is just incapable of anything.
That’s a metaphor for all Beyer’s, isn’t it? SHOTS FIRED! 
Hey now, I like beyers I take exception to that one 
then again even I would agree 177x isn’t all that uhh.. great
