It's a really tough question, because I don't have a lot of regrets about where I've ended up, and if I had made a different decision at this point or that, I'm not convinced that I would have been mature enough to take advantage of it anyway.
If I had to pick one, it would be a phone call that I never returned from a really good friend from high school. We had pretty much torn it up when we hung out, so he was basically my best drinking buddy. I hadn't heard from him in 20 years and I had just gotten married (2008) and had an 8 year old son. He was a very good dude, but from his personality, I would have guessed that he was still the same guy then as he was in high school.
We were on the way out the door and he called and left a message, and then another one and then another one in the span of a couple minutes. There was nothing particularly urgent about the messages, but leaving three in such a short span kind of weirded me out. My life had just calmed down significantly, and I guess it just felt like a bad time to introduce a chaotic element into it, so I didn't call back and then kinda forgot about it.
About two years later, it was weighing on me, so I decided to try to find him and give him a call. Talked to a friend who had been in touch with his brother, so I got ahold of him. Turns out that he had just passed away from a heart attack while helping his brother move. I think he was maybe 41? Damn, I wish I had made that call.