Mine would be:
Not spending more on a better build quality psu (not because of reliability or some nonsense though).
My psu fan has always been the loudest part of my computer
Not spending either 100 euros more on a quad core phenom II or 170 more on a i7 920+intel mobo
I bought a 3 core phenom II which was the best bang for buck cpu EVER and it was dirt dirt cheap (it was a fully featured 955BE with one core disabled but the cache fully intact for half the price) and served me well for 3 years , but I assumed cpu performance would actually go anywhere in the next 5 years (I was wrong, thanks AMD)
So now it's been time to upgrade for a while and the only real upgrade path is a brand new intel mobo + i5 + ddr3 ram (my am2 mobo uses ddr2) which is like a 400 euro upgrade...
If I had bought an i7 920 back then I'd have had been a lot better off in natural selection 2, saints row 3-4 and a bunch of other games and I'd still be good for performance today
I really don't want to pay 400 euros for an incremental poor build quality upgrade over the i5 2500k with shitty Thermal paste
Alternatively I wish I had just bought an i5 2500 k when it released, that would have been good too even though my phenom II was still fine back then
Not buying a sony fw900 back in 2003
my IIYAMA was a pretty high end monitor (and not cheap) so it's pretty good but with crts going the way of the dodo and LCD monitors being guttertrash I have no upgrade path or even sidegrade path and fw900s go for 1000+ euros now
My IIyama is starting to break (some horizontal lines at the very top of the screen and the screen geometry got worse) and I have literally no way to replace it without a huge downgrade
Buying a 32GB raptor hdd in 2004, I never even installed the thing because I was too lazy to reformat and kept putting it off, one of my only impulse buys ever
Going amd again with my hd6870, it was meant to be a cheap tie me over card until I got a brand new build sometime in 2012-2013 when 28nm cards were out and when ivy/haswell released but see above as for why that didn't happen
Now there is no point upgrading my gpu with this cpu and I had to suffer with really bad amd drivers between 2011-2013 and with no way to downsample on my crt monitor...
but hey hindsight is 20/20 and the phenom II was the best hardware deal I ever found, I could've never guessed what would happen in cpu land or how shitty bulldozer would be
Next time i'll definitely buy a quiet psu and intel cpu, and keep praying that OLED monitors appear sometime before 2020...