I am lucky enough to get proper mentorship early, so I didn't make too many gear mistakes, at least I didn't think so. Still here is a list of things I have bought, which I regretted. These only reflex my personal experience. they may not apply to everyone.
The first group, I got zero value them. Once or a few uses, regretted buyin
1. My first regulator: Oceanic FDX10 + Delta4 with swivel. Swivel failed on 1st dive. Brought in Oceanic HQ in San Leandro for repair. During the repair, a recall was found for the 1st stage. They had to fix the recall or they couldn't ship the reg back to me. Costed me ~$100. The recall was free, but in order to get to the recall part, they needed to do a service on a reg after 1 dive. 2nd stage did free flow after the "service". After a few adjustment without success, I gave up. What I also learned is Delta4 2nd stage is NOT balanced. The so call "mechanical balanced" is not balanced at all. There is no balanced chamber. Knowing what I know now, I think a perfectly tuned Delta4 will free low at depth with low tank pressure because it is where the IP is the highest. You can always detuned the Delta4 to alleviate the issue. To me, it is a bad combination.
2. My 2nd regulator: Hollis DC2 212 with swivel. (I was a sucker for swivel before I know better). Problem 1: DC2 first stage ports are so close, a wireless transmitter (Oceanic in this case) can't be attached without interfering other LP hose. I was forced to use a 6" hose for a transmitter. Problem 2: LP ports are recess. LP hose fitting without some tapper before the threaded part will NOT fit. The long hose I ordered didn't fit. Problem 3: The 2nd stage swivel can NOT be simply removed and replaced by jam nut. To remove the swivel, the entire air tube needs to be replaced. Hollis/Oceanic quote me $150. According to them, anytime a reg come in and needs to be taken apart, a service will need to be done. At that point I lost hope in their regulator. I did open up 212 to see what it is inside. Compare to a HOG 2nd stage that I used as octo, I can honest say 212 build quality is far sub par compare to HOG 2nd stage.
3. Big dive knife. After #4, I bought a big 6" Ti knife from Atomic. Never once I carry it because it is too big. I use it at home now for random stuff.
4. Expansive dive knife. It is a smaller 4" blade Ti BC knife, with locking plastic holder suppsed to attached to BC. I carried it for some dive, then the locking mechanism failed, and I lost it.
5. DiveRite 16lb QLR pockets (8lb per side), bought them for Transplate harness. Pockets are way too big. If you put less than full capacity in, say 4lb, the weight will move around. The opening for the drain is also way too big. A 2lb bullet will slip out with some movement.
6. Octo holder of any kind. Tried a few, none works properly when you need them to.
The next group, I got some usage/value out of them, but if I were to start over again, I won't buy again
1. Fancy fins: My first fins from OW, AL slingshot, bought them because the instructor was using a pair, so they must be good right!!! ~$175 was what they costed back them. They are OK, but way too expansive for what they do.
2. DiveRite crotch strap. The stiffest webbing I have encounter, DR choose them for crotch strap. Even DR throw in neoprene sleeve, I can still feel the stiffness in that area.
3. Oceanic swiv pressure gauge. All platics, low built quality. I got it together with Oceanic FDX10/Delta4. Should have just got plain brass gauge.
4. DiveRite transplate harness. It is OK, wasn't as comfortable as it looks.
5. Wireless AI computer. I got it for the coolness. As my diving progress, I learned I can't rely on the ATR. I also don't need to look at the pressure reading every seconds. So one day I removed it and never miss. It is sitting in my dive closet now.
Again personal experience and preference, sorry if I offended anyone or any manufactures