Oh man, is it confession time again? I thought we already had one of these threads.. sigh.
Well, I've already confessed one of the worst things I've ever done (to myself), in the other thread. The whole blacking out and losing my virginity in New Orleans trip... yeah.
Probably the next worst thing was what I did to someone else, once. There was this guy R, who was in love with me our first two years in college... and to whom I had absolutely no physical attraction, but I could really "see myself marrying" (go figure; I was a clusterfucked evangelical back then). He was the "godliest" person I knew, and I respected no one more than him. My problem was that I never wanted to kiss him... I just wasn't attracted, but kept trying to make myself like him. I suppose, in today's terms, he was my cuddle bitch.
In any case, he told me his feelings towards the end of that school year... at a Starbuck's near campus (one of our hangouts). I still remember the amazing things he said to me, about me, across that table at Starbuck's. I had never heard anyone talk like that to me before, with such sincerity and honesty and true attraction. He truly was the kindest person I had ever met... but I felt nothing for him other than friendship.
And I had to tell him that, to his face... and tell him that I didn't want to date anyone before leaving that summer (I was going to Thailand for 2.5 months). I left there, parked my car at my dorm, and cried for a long time. I felt so awful for not reciprocating, especially because I had been in an unrequited love situation myself, not too long before that. I hated having to do that to another human being.
You'd think that was the worst part. No. During that same quarter, I developed a crush on this hot guy on my crew team (P)... and I REALLY wanted to kiss him (I was 18 and had never kissed anyone before)! The feelings ended up being mutual and P asked me on a date just before I was to leave for Thailand for 2.5 months that summer. I told one of my close friends about it, but that was it.
A day or two after the date, R drives like an hour to my place to say goodbye (even though I've just broken his heart a bit) the night before I leave for Thailand. We sit and talk, and then my aunt barges in and says, "How was the date with P?", totally oblivious to the fact that R is sitting there, still nursing his heart, especially after I had told him I "didn't want to date anyone before leaving."
I deny the whole thing, say, "What are you talking about, dear aunt??? You must be thinking of that... uh, other niece of yours [the one who doesn't fucking LIE to nice people]." I was totally freaked out, backed into a corner, was trying to protect both myself and R's feelings (somehow). We pretended like it was all fine, but it felt awkward and R left shortly afterward.
My close friend calls me, the one I had told about the date with P... and tells me that she mentioned the whole thing to R earlier (she thought R already knew, apparently). So that WHOLE time, R knew that I was lying to him... and he never said a word. He just left the house. I told my dad about it immediately, and he told me to call R *NOW* and apologize. I did so, as soon as R got back home... but it was awful. R said he forgave me, but I knew he resented me for the next 1.5-2 years. I felt so very awful, even though I ended up with P and didn't regret that happening.
Eventually, things got back to normal with me and R. P and I broke up after about a year, and R ended up with his first girlfriend.. nearly got engaged to her (she turned him down). He went through that heartbreak and went on to be a great elementary school teacher. And as far as I know, he was single for the past 6 years up until this spring. He met a wonderful girl and got engaged, and included me in the group e-mail announcement about that.
I felt honored that he would let me know... I still feel guilty, sometimes, for lying so very painfully to him that one time, even though I was just 18 and had no clue what I was doing, emotionally/relationally. But I am glad he is happy now, and found someone who truly loves him and is attracted to him.

And ever since that moment, I have sworn that I will not lie again, not even to try and "protect" someone's feelings... because essentially, it comes down to selfishness and protecting ME from getting hurt. Which doesn't work.