My main source of regret w.r.t. gear is more about an overall process than tied to any one particular item. The process is likely familiar to many, it's incrementally (and expensively) inching your way towards what turns out to be your ideal pack base weight.

It's where you buy what seems like a nice sleeping bag, then a few years later buy another one that's explicitly lighter or warmer (or both). Then when you get more serious about lightweight backpacking you buy a third one. Double the above in terms of storage space at home and cost if your spouse backpacks.

Then expand the concept to shelter, backpack, clothing, and various other gear.

A lot of potential for regret there. I think it's really hard to jump right to where you'll end up wanting to be, but perhaps I could have skipped an (expensive) intermediate step on a lot of gear.