I have gone through a LOT of crappy gear. But I have only really regretted a few things. Most of that crappy gear is just stuff I outgrew. Still not done outgrowing things either I guess. I just got rid of a Neumann TLM 103 because it was finally just not doing anything for me anymore.
Stuff I completely regretted buying:
-MXL 4000 (sounded like a toy)

I actually traded a Cascade Victor mic for that thing and I really regretted that. I was naive.
-Monoprice audio cables. They are pretty cheap but also pretty crappy.
-Presonus FirePod - Sorry I know a lot of people use them but as soon as I got it I knew I was going to have to keep searching for a different interface. Sounded flat, dead, icy, whatever. I just couldn't stand it.