I have a few regrets:
Worst Trade I made is realized in retrospect: I traded a Yamaha BB415 for a MIM Jazz back in 2006. At the time, I felt like it was a decent trade. After owning several Jazz basses of different quality margins, even other MIM jazz basses proved better than the 2006 bass. The Yamaha was a good bass that I wish I still had.
Badass Bridges: They don't sound better, they may function a little better, but they certainly weren't worth 100 dollars. They never made that big of a difference to me.
Gotoh 201 Bridge. It experienced tail lift after a couple months and the oversized saddles never got low enough.
Pickups: If your bass sounds like crap, in my experience its probably not the pickups (if they are a traditional design that is). Tried 4 different sets in a crappy sounding MIM jazz I have. It sounds lifeless with any set.
Seymour Duncan Quarter Pounder Precision Pickups, god what a terrible pickup; overly loud, lacking in mids and aesthetically ugly.
Vintage basses: Sure some are really nice, but the prices people ask for some pieces of wood are ridiculous. You pay for the age, thats it. Especially when you realize that they aren't any better than the basses of today, with the possible exception of available woods in vintage years. I have seen some terrible looking vintage basses. Most of the vintage fenders i've tried or owned look and feel more imperfect than most modern ones. Mid to late 70s fenders are terrible, I don't mind saying so. Currently own a 71 P, its great, but I also got it for a great price. I'd rather buy a used CS in most cases.
Pedals: Oh god, so many terrible pedals out there, heres a couple I've tried; vintage DOD pedals, on bass they are crap. Terrible buffer and tone suck, not to mention the 1/8" power jack. Collectors overhype and overprice them, they are not that good. I currently own or have owned the envelope filter, phasor, milkbox, meatbox, juicebox and delay. Delay was the only good one and I like the envelope filter but its quirky.
DOD Meatbox, bought it after the Juan Alderete hype, soon realized that if I wasn't playing backline equipped stadiums, I wouldnt hear it. Sounds like a terrible octaver without the PA support.
Any Digitech pedal.
One Control Blue 360, a pedal supposed to be modelled after the Acoustic 360 Preamp. Sounds nothing like a 360. A two channel pedal that allows access to the other channel via a manual switch on the side of the unit. I never understood that. Plus the fuzz channel could never achieve unity gain, way too low with the volume maxed. No manual, tried contacting the company through email several times for help, never got a reply. cost 200$.
MXR Bass compressor. Sterile sounding and had to be replaced twice due to faulty electronics. Mine is a brick now. I liked the size and controls, but for a similar price, the Empress is way better.
Elixir Strings: I like the idea of long lasting strings. I don't like the idea of those strings sounding like 2-week old slinkys for a year. 50$ CDN a pack, no thanks. DR's are 20$ cheaper and last a long time when you have multiple basses.
I'll add this: I've never owned 'boutique' basses, because I simply don't think they can do what a precision or jazz bass can't. Once worked at a high end music store, and tried some 'amazing' instruments. 9000$ Ritter Royas, Foderas, F Bass, Ken Smith, all to be owned and eventually circulated in the used market. Ritter basses are awful, god they sound terrible and feel terrible why anyone would pay for one.... I was talking to an recording engineer once and he was telling me that boutique basses are proven or disproven in the studio. In most cases, a precision or jazz will do the job better.