My regrets:
5-6 masks looking for the magic non-leaky mask before realizing my wife is a nose-breather.
Tanks
Lobster bags. I used them once collecting megalodon teeth. They're still hanging on the wall years later.
Velcro-strap wrist slate. As has been stated, there's no good way to strap it on. Suit compression results in it flopping around or poor circulation to your hand. Even worse, the provided pencil will not write on it! EXCUSE ME! Test your product before shipping?! Duh.
Oceanic Caribe Fins. They're great fins. Problem is, we always wear boots. So the bare feet fins never get used.
Small lights. Hardly ever use them.
Items I don't regret similar to products seen others complain about:
Atomic SS1: I've used this occasionally and still love it. What's wrong with the Octo+ ? I have never regretted the SS1 for any reason.
Tusa Zooms. If you can't fight current with these on, learn how to kick with them. No challenger has yet been able to keep up with me when I'm using a short swift knee-bend kick. zooooom is absolutely right. With very little effort, I go so fast, my reg hose is tugging and flapping in the current. Back kick? Frog kick? Pffft. maybe if I ever needed those kicks.