At the end of the day, I find myself running through things that happened during the day, reliving and analyzing them. On the second time through, I often will think, ah, I shouldn't have said that, or not that way. I meant to convey X but made someone feel Y.
Here are some examples. At my previous job, I once made a passing joking comment about the relative youth of someone who was very bright and only 21, at a table of our peers. The larger context is this table of peers regularly talked shit about other teams amongst themselves, I was not thrilled about it, and this punk kid(!) was kind of their ringleader. But also, I wanted to fit in. Anyway, in the moment, this guy didn't seem terribly insulted, but I sensed he wasn't excited to have his young-ness brought up. This bothered me more when I thought about it after work—I imagined he'd gotten years of flak for being 'a kid', because he dropped out of college and started working, so that topic must be especially old and irritating; and because he looks older some people might not have known, so now I've maybe "outed" him to some of his peers. It's possible in the process of thinking about this, I made myself upset to a degree he never was.
Another example: just yesterday, in the course of a longer conversation in which I was getting impatient, I told someone who was proposing some work to do that "I'd rather be doing something else." Thinking back on it, I realized that might have been insulting, and is not even what I had actually meant, which was "I recognize this proposed work is valuable but I think there are more pressing things right now."
I've started following up on situations that seem to matter (as in both of the above), and technology has made that easier with messaging. Easier than pulling a person aside the next time I see them, whenever that might be. It's a godsend for a someone who finds it challenging to think and talk at the same time.
There are some situations that happened longer ago that I have never resolved, that were particularly cringeworthy. They come up and bother me every so often. Most of these, I can put to rest by diagnosing them. There is a meeting with a director at Facebook I wish I could do over. (This one actually might be worth writing about at a later point, because there's a larger observation about growth here.) An interaction at a career fair with a cofounder of one of the most successful tech companies is my most embarrassing story. (The only real lesson here is to not cave to peer pressure to talk to anyone and that I need to be extra prepared when talking to total strangers.) But this morning I was thinking about a situation that was more mundane, which I still don't have a good resolution to. I think it falls into a category of things that are just better not said, but only as adults. 
A few years ago, I told an African American acquaintance, a male coworker, that I remembered him and liked his hair. It wasn't the first comment, it was the second. He had a fro that had made an impression on me, which seemed to fit him well. I guess the thing I immediately realized is, how uncomfortable and racially charged that compliment is, when it's from an Asian woman to an African American man. It might not even be so bad if the coworker had been a woman, because it's more normal for women to compliment each other's hair even if they don't know each other well. The thing that stops me, though, is that I think it wouldn't have been weird if the coworker had been of nearly any other race, but still a man. It would also have been much less weird had I been five years old and seen as "innocent." I've wondered if there was a different wording that I could have used, but I don't think so. The lesson there was to just stick to compliments about what people do, or... at most their outfit.
 Which reminds me of http://www.paulgraham.com/say.html