The original clip that came with my very expensive ScubaPro dry snorkel would not stay on my mask strap. After the snorkel went to the bottom, my dive shop guy told me that happens all the time; the clip-over-the-strap holders never work. He sold me a much better snorkel strap for a couple bucks. I asked why he hadn't suggested that when I bought the snorkel; he said he didn't want to push me into buying more stuff. After having bought the entire "Total Diving System" from him, I think a couple bucks to save an expensive snorkel would have been worth it.
On the other hand, I bought a comfort strap that would not hold my (also expensive) ScubaPro mask to my face because the strap kept slipping through the buckle, which was not a problem with the original strap. It too took the plunge even after the divemaster checked it and said it was OK. Trying to get on the boat in 6-10-foot seas with no mask was not fun. But in retrospect, I'd had problems with both of them and feared they might fail. I should have addressed those issues ashore before any of that happened.
Let's see: Scuba.Com sells straps that allow you to carry your tanks like a suitcase. I bought three and have never used them. Everyone just carries tanks by the valves. I didn't know that when I was new and eager to buy up cool scuba stuff.
Finally: Scuba Goop. This is gear marker paint available on scuba.com for $6 plus shipping. It arrived with another label underneath the "Scuba Goop" label. Turned out to be "Scribbles 3D Paint" available and any local crafts store for about a dollar. Yeah, I regretted that one. Scuba.com removed my review from their site.
Things I wish I would have bought sooner: My dive skin, for jellies in warm water. A cheap (but much better) snorkel holder. A cheap (like six bucks) BC knife from Scuba.com that actually stays in the plastic sheath. My cool but -- guess what -- very expensive ScubaPro dive knife also joined Neptunes Thrift Store because all you have to do is press it slightly (like leanijg your BC against another BC) to release it from the sheath.
Anyway, that's all off the top of my head. Good thread.
It is a toss up between my snorkel and my wife. 
