Lazyass

I regretted buying my carbon disc road bike (Diamondback Podium) because the 105 brakes squealed so loud it was embarrassing and no tip or trick I found on the internet would silence them. It was like an airhorn going off, people would stare.

When I was on the verge of selling the bike and staying away from discs forever I came up with the idea to sand the rotors. They've been totally silent ever since but they never felt any more powerful than my rim brakes and didn't have the bite I thought they should. They were nothing like the SLX's on my MTB, those things are so powerful it's almost dangerous. So I was okay with them but never loved them.

When I hit about 3500 miles on the bike they suddenly became more powerful like I thought they should have been, it happened almost overnight. Now they're great. I guess they need a 3500 mile break in

DMC707

I didn’t pay retail , but a team I was riding for was using Giant TCR’s in the early ‘00s

I never could get along with that bike. The S, M, L frame sizing was problematic The “pro fit” top tube was just too long, the carbon aero seat post made for a jarring ride (replaced it with a Dura Ace post and problem solved though)
The bikes were dead ringers for the ONCE team bikes though and looked great!

but was pretty happy when we got a deal on another brand the next year

Flatforkcrown

My de Rosa slx. Worst $500 I ever spent. Who even likes old Italian steel bikes anyways?

just kidding… I only regret the bikes I didn’t buy.

Ghazmh

Onetime I bought clip on fenders that fit a 25mm tire. Totally useless.

Hiro11

1. SRAM hydraulic MTB brakes up until about three years ago were unreliable dog poo.
2. Buying complete bikes with heavy OEM wheels and unbranded hubs. OEM wheels are generally garbage, I'd happily spend a little more for something better.
3. Lightweight carbon wheels. I only weigh about 165 and I've now broken three rear spokes in about 18 months on three different (high end carbon racing) wheelsets. Breaking spokes in general seems more common than it once was, i think they're really riding the line with durability these days on fancy wheels. I'd rather have a few more spokes and much more strength.
4. Most saddle bags I've purchased are annoying garbage.
5. GP4000 and (non-tubeless) GP5000 tires have sidewalls that will suffer ride ending cuts if you look at them wrong.
6. $150-$200 high end seatposts from Deda, Ritchey and Thompson I've purchased over the years. I swear, the $28 carbon seatpost I bought from Amazon two years ago for my gravel bike is just as light, just as durable and has a better designed and easier to deal with rail mounting system.
7. Carbon stems are a total waste of money compared to high end aluminum stems.
8. Carbon mountain bike wheels are a total waste of money compared to high end aluminum wheels.
9. XTR SPD pedals aren't as durable as the M540s which cost a small fraction of the price. The weight difference isn't worth it.
10. POC helmets are beautifully made but overpriced and don't fit me at all.
11. Gloves with padding don't work for me at all.
12. Hincapie custom kits are expensive garbage. Castelli custom fits better but is not durable at all. Starlight is far cheaper than both and significantly better/more durable.

Kapusta

I only have regrets for things where I should have known better.

So while I think it was a bad choice to buy the Magura MT7 brakes last year for my MTB, I can’t say I regret it.

But what I DO regret is throwing good money after bad by buying a new lever when the first one was toast. I should have just cut my losses.

DLeeW

Regret my most recent purchase, a couple of months ago. It is, as near as I can tell, an '81 Made in Taiwan Raleigh Reliant Mixte. Didn't really expect much from it and I didn't spend much for it, but It is the twitchiest bike I have ever ridden. The thing is almost like new, must have been hanging in someones garage or shed for all these years. Part of the problem may be that I have been used to riding a couple of touring bikes. But then one of the best rides I've had was an '82 or so Peugeot Mixte, so I was hopeful on this Raleigh, took off the drop bar, put on a cruiser type, didn't help. Just not a design that works for me, maybe I'll get used to it. I have been lucky, of all the bikes I have had over the years I can say I liked them all, until now.

Nachoman

I bought a tandem one time when my kids were little. I purposely bought one that was too large, figuring they would grow into it.
But it took a couple years for them to grow into it, which basically meant a couple of years of suffering with the wrong size tandem.