Ask HN: What is the thing you've built that you regret the most?

Given the very interesting comments on the "Ask HN: What is the most impactful thing you've built?", I was wondering about something similar: Things you regret based on ethical implications, bad technical decisions you made convinced you were right but regret/cringe about later, failures on miscalculations on budgets that provoked a bad outcome in the company etc whatever.

Thanks in advance.