A few things come to my mind: during my very first relationship, I developed feelings for my DP's brother. Nothing ever happened but DP found out about my feelings in a really unexpected way and he was absolutely gutted. I was quite young but still, I feel awful about it now.
I also led someone on who I knew had feelings for me. I knew I didn't return their feelings but I unintentionally kept them on the backburner because I was scared of being single. I'm pretty sure he still hates me now even though it's been many years.
I'm sure there are other things, but those are a couple that come to mind. What's the worst thing you've ever done?