hmmm....my latest batch of regrets:
Large Jet fins. Right now, I own a set of medium, large, and x-large Jets. The mediums work well for my wetsuit/single tank diving, and the x-larges work great for my drysuit/doubles diving, but what to do with the larges? They're too big to use for wetsuit diving, and they don't provide enough propulsion for double tank diving. Guess they are doomed to a life of sitting on my "extra gear" shelf
A wetsuit for my non-diving fiance'. It seemed like such a good idea at the time: Let's get the fiance' into diving! So, I went out and bought him a 3mm wetsuit...then the guy had to go and lose 60 lbs. Now, I've got a XXXL wetsuit, and no one to put into it. "Extra gear" shelf it is!
A 2002 SE Miata. I love the car, but you try cramming doubles, a deco bottle, a drysuit, two gear bags, a spares kit, a small Pelican case, and two reg bags into a car the size of a sardine can!

Really starting to wish I had replaced my wrecked '92 Miata with something that has cargo room...say, a Range Rover.